The Buddha explains why he teaches only a little compared to what he has not taught with the simile of the rosewood leaves.
Sīsapāvana sutta - Rosewood Grove
Once, the Blessed One was dwelling in Kosambi, in the rosewood grove. Then, taking a few rosewood leaves in his hand, the Blessed One addressed the bhikkhus: "What do you think, bhikkhus, which is more numerous — the few rosewood leaves that I have taken in my hand, or those in the rosewood grove above?"
"Very few, venerable sir, are the rosewood leaves that the Blessed One has taken in his hand; indeed, those in the rosewood grove above are far more numerous."
"In the same way, bhikkhus, what I have taught you is only a little compared to what I have not taught you. Why have I not taught it? Because it is not beneficial, does not relate to the fundamentals of the spiritual life, and does not lead to disenchantment, to the fading of desire, to gradual ending, to tranquility, to direct knowing, to full awakening, to Nibbāna. That is why I have not taught it.
And what have I taught, bhikkhus? 'This is suffering,' bhikkhus, I have taught; 'this is the arising of suffering,' I have taught; 'this is the ending of suffering,' I have taught; 'this is the way of practice leading to the ending of suffering,' I have taught.
Why have I taught this? For this is beneficial, relates to the fundamentals of the spiritual life, and leads to disenchantment, to the fading of desire, to gradual ending, to tranquility, to direct knowing, to full awakening, to Nibbāna. That is why I have taught it.
Therefore, bhikkhus, effort should be made to fully understand: 'This is suffering'; effort should be made to fully understand: 'This is the arising of suffering'; effort should be made to fully understand: 'This is the ending of suffering'; effort should be made to fully understand: 'This is the way of practice leading to the ending of suffering.'"