I believe the answer to your question is given in "The Brahma's Request" quoted below.
As it reads, the Buddha taught out of compassion / mercy to beings almost free from the dust of worldlings.
The Buddha refrain from passing into the final Nirvana after a request by Brahma Sahampati.
THE BRAHMA'S REQUEST
The Blessed One having attained Buddhahood while resting under the
shepherd's Nigrodha tree on the banks of the river Neranjara,
pronounced this solemn utterance:
"How sure his pathway in this wood, Who follows truth's unchanging
call! How blessed, to be kind and good, And practice self-restraint in
all! How light, from passion to be free, And sensual joys to let go
by! And yet his greatest bliss will be When he has quelled the pride
"I have recognized the deepest truth, which is sublime and
peace-giving' but difficult to understand; for most men move in a
sphere of worldly interests and find their delight in worldly desires.
The worldling will not understand the doctrine, for to him there is
happiness in selfhood only, and the bliss that lies in a complete
surrender to truth is unintelligible to him. He will call resignation
what to the enlightened mind is the purest joy. He will see
annihilation where the perfected one finds immortality. He will regard
as death what the conqueror of self knows to be life everlasting. The
truth remains hidden from him who is in the bondage of hate and
desire. Nirvana remains incomprehensible and mysterious to the vulgar
whose minds are beclouded with worldly interests. Should I preach the
doctrine and mankind not comprehend it, it would bring me only fatigue
Mara, the Evil One, on hearing the words of the Blessed Buddha,
approached and said: "Be greeted, thou Holy One. Thou hast attained
the highest bliss and it is time for thee to enter into the final
Then Brahma Sahampati descended from the heavens and, having worshiped
the Blessed One, said: "Alas! the world must perish, should the Holy
One, the Tathagata, decide not to teach the Dharma. Be merciful to
those that struggle; have compassion upon the sufferers; pity the
creatures who are hopelessly entangled in the snares of sorrow. There
are some beings that are almost free from the dust of worldliness. If
they hear not the doctrine preached, they will be lost. But if they
hear it, they will believe and be saved."
The Blessed One, full of compassion, looked with the eye of a Buddha
upon all sentient creatures, and he saw among them beings whose minds
were but scarcely covered by the dust of worldliness, who were of good
disposition and easy to instruct. He saw some who were conscious of
the dangers of lust and wrong doing. And the Blessed One said to
Brahma Sahampati: "Wide open be the door of immortality to all who
have ears to hear. May they receive the Dharma with faith."
Then the Blessed One turned to Mara, saying: "I shall not pass into
the final Nirvana, O Evil One, until there be not only brethren and
sisters of an Order, but also lay disciples of both sexes, who shall
have become true hearers, wise, well trained, ready and learned,
versed in the scriptures, fulfilling all the greater and lesser
duties, correct in life, walking according to the precepts-until they,
having thus themselves learned the doctrine, shall be able to give
information to others concerning it, preach it, make it known,
establish it, open it, minutely explain it, and make it clear-until
they, when others start vain doctrines, shall be able to vanquish and
refute them, and so to spread the wonderworking truth abroad. I shall
not die until the pure religion of truth shall have become successful,
prosperous, widespread, and popular in all its full extent-until, in a
word, it shall have been well proclaimed among men!"
Then Brahma Sahampati understood that the Blessed One had granted his
request and would preach the doctrine.