Because, like most belief systems Hinduism encounters, it simply ate it. Hindu theology is very malleable, so porous and fluid that it can embrace virtually any spiritual belief or practice as one of its own. While many other religions would have confronted and challenged the Buddhist growth directly, Hinduism absorbed it. The question of whether or not Buddhism is another offshoot of Hindu belief is still a valid one. As such, there is little motivation for Hindus to convert to another faith. (As the Christian missionaries quickly found out.) There is virtually no religious/spiritual claim one can make that Hindus do not readily agree to....except for the absolute ones. (Jesus is the ONLY way to the Father; Mohammad is the LAST prophet of Allah, we are THE CHOSEN people of God, etc.) As Sri Aurobindo defined it, "India is the meeting place of the religions and among these Hinduism alone is by itself a vast and complex thing, not so much a religion as a great diversified and yet subtly unified mass of spiritual thought, realization and aspiration."

That said, the component that did not return after Buddhism was Brahmanism. It was this elitist structure that the Buddha (and the rest of the sramana teachers) opposed. But as in the past with its animistic elements and to the present with the crumbling caste system, the people of the Indus shed the weaker, dying bits in favor of persevering and strengthening.

Or, from a more historic perspective, it can also be attributed to the Muslim invaders that saw Buddhism akin to atheism and therefore not bound to the relatively restrained Quranic rules of engagement. In 1193 the Buddhist decline began with Qutb-ud-din-Aybak's invasion and only increased from there under the Mughal rule. Combine this with Hinduism's spiritual absorbency and you get a pretty good idea why Buddhism faded from the subcontinent and Hinduism remained relatively unscathed.