Bearing in mind that ignorance is in opposition to knowledge and the starting point for all the suffering, it seems still unbelievable that there were people contemporary to the Buddha that contradicted and refused his teaching. Considering that he was the culmination of human intellect and the embodiment of pure knowledge, how is it possible to be in such stubborn opposition with the truth?
Basically, because of the ten fetters (samyojana) that bind one to the cycle of neverending becoming (Samsara). And since ignorant people defend and strengthen those fetters instead of weakening and breaking them.
1. sakkāya-diṭṭhi - individualistic or subjective views
2. vicikicchā - skeptical doubt
3. sīlabbata-parāmāsa - valuing false systems of morality
4. kāmacchando - desire for sense pleasures
5. byāpādo - malevolence
6. rūparāgo - craving for matter or form
7. arūparāgo - craving for something immaterial or formless
8. māna - pride or conceit
9. uddhacca - restlessness
10. avijjā - ignorance
The breaking of the first 3 fetters equates to a stream-enterer (Sotāpanna). By weakening the 4th and the 5th one is a once-returner (Sakadāgāmin) after which by breaking them one is a non-returner (Anāgāmi). And after discarding the next 5 fetters one becomes a fully enlightened being (Arahant).
It could have happened because those people did not believe that enlightenment is possible or because they had a false impression about what is meant by enlightenment. In both of these cases, they would have considered the teaching of the Buddha yet another set of opinions, unaware of what true knowledge really is.
There's also this: Gautama said there is this thing called complete enlightenment and once you attain it, you're all good. However to attain it you have to apply rigorous skillful means that are best applied under the tutelage of a master. Said master will probably require you to work, earn money for your upkeep, etc. In short, it's not too much different from certain modern-day preachers who might set up camps where people are made to study a holy book or two all day and work and give ALL they earn to the "community", collected by the preacher. See the issue? The world it seems has always had people in it who claim to have THE WAY and if you follow them and/or their delegates, you will gain some kind of reward that is very hard to attain and it also pretty hard to demonstrate as actually existing. Point to "enlightenment". Point to "nirvana". See what I mean? So that there were skeptics back in the day of the Tathāgata should come as no surprise, just as there are plenty of skeptics today who challenge preachers who claim to have THE solution to all Man's problems, for a price of course.
Now I know there's no record of Buddha saying for a price, he'll teach you how to meditate and become enlightened, if you work hard enough. But there are records indeed of members of his following, especially post-his-demise, suggesting that for a price AND the right effort, nirvana awaits. Even if Buddha were utterly pure of heart and intention in his teachings, these other types of people living in that age perhaps preaching something different, though preaching nonetheless, could have sullied the waters and made fertile ground for skeptics to be skeptical.
A few bad apples ruin the barrel.