For what reason did Buddha rejected Upanishad and Vedas?
Because nibbāna of Upanishad and Vedas is sassata-diṭṭhi (eternalism) in brahmmajālasutta. But nibbāna of buddhism is enlightened by ariya at the finish line (enlightenment) of the middle way, that is in the midst of whole extreme teaching around the universe, such as eternalism, annihilationism, egoism, altruism, theistic determinism, determinism that only past-life's karmma is cause of present life (i can't find the correct word of this view), accidentalism, nihilism, Marxism, democrazy, communism, science, capitalism, etc.
What is the middle way?
The middle way is ariya-magga (The Noble Eightfold Path), the best balanced teaching which must leads to the complete and perfect whole suffering cessation (anupādisesa-nibbānadhātu). The middle way must have these whole properties as their results, not just some properties:
By avoiding these two extremes, O bhikkhus, the Tathāgata has gained the knowledge of the middle path which leads to insight, which leads to wisdom, which conduces to calm, to knowledge, to the Sambodhi, to Nirvāna.
‘This, O bhikkhus, is the Noble Truth of the Cessation of Suffering: (it ceases with) the complete cessation of this thirst -- a cessation which consists in the absence of every passion -- with the abandoning of this thirst, with the doing-away with it, with the deliverance from it, with the destruction of desire.
‘And this knowledge and insight arose in my mind: "The emancipation of my mind cannot be lost; this is my last birth; hence I shall not be born again!"’
“Monks, the learned, noble disciple seeing it thus turns from the eye and forms, eye-consciousness, eye-contact and whatever feelings, pleasant unpleasant or neither unpleasant nor pleasant born of eye-contact, he turns from that too re He turns from the mind, deas, mind-consciousness, mind-contact and whatever feelings, pleasant unpleasant or neither unpleasant nor pleasant born of mind-contact, he turns from that too. Turning loses interest. Losing interest is released. Released knowledge arises to him, I am released, birth is destroyed, the holy life is lived to the end, duties are done, I have nothing more to wish.”
The Blessed One said thus and those monks delighted in the words of the Blessed One.
When this exposition was done about a thousand monks released their minds from desires without anything remaining.
"These are the single gold paths to the purification of beings, to the overcoming of sorrow & lamentation, to the disappearance of pain & distress, to the attainment of the right method, & to the realization of Unbinding — Summary are the foundations of mindfulness.
Tipitaka studing that heritage from Arahanta, tipitaka reciting, and tipitaka memorizing his tipitaka knowledge, these 3 things make the buddhist practitioner understands tipitaka.
Reference to the Upanishads does not exist in the Pali suttas. Only reference to the Four Vedas is found in the Pali suttas. The Buddha was not aware of the Upanishads and had no interest in the superstitious ideas found in the Four Vedas.
The Buddha found the complete cessation of suffering, which is not found in the Vedas. The complete cessation of suffering occurs with the utter destruction of "self-view", which includes the destruction of ideas about "Brahma" and "Atman".
A few Upanishads are pre-Buddhist but the majority came later. Most of the Vedas were however pre-Buddhist.
The Buddha definitely rejected the idea of the Atman (an indivisible indestructible eternal self at the core of beings) and he also rejected the concept of eternalism. Atman and eternalism are core doctrines of the Upanishads.
Not just this, there are rituals that are either contained in or derived from the Vedas like the following (from this site):
In its simplest form, which is still practiced by many Brahmanas and members of the higher castes, it is practiced just before the dawn by standing in a water body such as a river or a water tank and offering water (tarpana) to the Sun as it rises in the east with a prayer and a salutation (vandanam), which makes the sacrifice both an external and an internal cleansing worship.
In the Sigalovada Sutta, the Buddha rejected the ritualism behind the Vedic bath prayer ceremony (as above) practised by a young householder by the name of Sigala, and put a new spin on it, according to the Buddha Dhamma.
Also, from the Canki Sutta, the Buddha hints that even the learned brahmans themselves are unsure whether the Vedas are true, by their own empirical validation:
"And among the brahman seers of the past, the creators of the hymns, the composers of the hymns — those ancient hymns, sung, repeated, & collected, which brahmans at present still sing, still chant, repeating what was said, repeating what was spoken — i.e., Atthaka, Vamaka, Vamadeva, Vessamitta, Yamataggi, Angirasa, Bharadvaja, Vasettha, Kassapa & Bhagu: was there even one of these who said, 'This we know; this we see; only this is true; anything else is worthless?'"
"No, Master Gotama."
"So then, Bharadvaja, it seems that there isn't among the brahmans even one brahman who says, 'This I know; this I see; only this is true; anything else is worthless.' And there hasn't been among the brahmans even one teacher or teacher's teacher back through seven generations who said, 'This I know; this I see; only this is true; anything else is worthless.' And there hasn't been among the brahman seers of the past, the creators of the hymns, the composers of the hymns... even one who said, 'This we know; this we see; only this is true; anything else is worthless.' Suppose there were a row of blind men, each holding on to the one in front of him: the first one doesn't see, the middle one doesn't see, the last one doesn't see. In the same way, the statement of the brahmans turns out to be a row of blind men, as it were: the first one doesn't see, the middle one doesn't see, the last one doesn't see. So what do you think, Bharadvaja: this being the case, doesn't the conviction of the brahmans turn out to be groundless?"
In the Sangarava Sutta, the Buddha rejected purification rites using water.
In the Sundarika Sutta, the Buddha rejected caste by birth.
In the Paccha-bhumika Sutta, the Buddha rejected the use of prayers to help the deceased go to heavan.
In the Samannaphala Sutta, the Buddha forbade his monks from doing fire oblations or fire sacrifices, practising astrology, reading omens, calculating auspicious dates for marriages, consecrating sites for construction and worshipping the Sun. Some of these practices are religious, while others are cultural.
I think the parable of the poisoned arrow is extremely relevant here:
When The Buddha was asked of the 14 metaphysical questions, he simply didn't respond to them. He said, for a man shot with the arrow, the important thing is to get rid of the arrow (the suffering) and not the material of the arrow or the angle it came from.
It's just as if a man were wounded with an arrow thickly smeared with poison. His friends & companions, kinsmen & relatives would provide him with a surgeon, and the man would say, 'I won't have this arrow removed until I know whether the man who wounded me was a noble warrior, a priest, a merchant, or a worker.' He would say, 'I won't have this arrow removed until I know the given name & clan name of the man who wounded me... until I know whether he was tall, medium, or short... until I know whether he was dark, ruddy-brown, or golden-colored... until I know his home village, town, or city... until I know whether the bow with which I was wounded was a long bow or a crossbow... until I know whether the bowstring with which I was wounded was fiber, bamboo threads, sinew, hemp, or bark... until I know whether the shaft with which I was wounded was wild or cultivated... until I know whether the feathers of the shaft with which I was wounded were those of a vulture, a stork, a hawk, a peacock, or another bird... until I know whether the shaft with which I was wounded was bound with the sinew of an ox, a water buffalo, a langur, or a monkey.' He would say, 'I won't have this arrow removed until I know whether the shaft with which I was wounded was that of a common arrow, a curved arrow, a barbed, a calf-toothed, or an oleander arrow.' The man would die and those things would still remain unknown to him.
That said, The Buddha also always paid respects (and made his followers respect) to all of the great masters of eternity, in the past, present and the future. So that would include those authors of Upanishads that presented their perspective of the reality.