That is because Hinduism absorbed most of Buddhist teachings into it's doctrine thus making Buddhism redundant.Apart from that Buddhism was never a majority religion in India.
You do understand that Hinduism is not an Abhrahmic religion and is open to change.
Original Vedic Hinduism did not had concept of rebirth. In Rig Veda, once you died you got to either Abode of fathers (Heaven) or House of clay (Hell). Concept of rebirth and cycle of life ,though developed by Brahmins, was a fringe belief in Hinduism. Buddha popularized it and it became dominant theology in Hinduism with passage of time.Similary is concept of moksha. Doctrine of non-violence was also borrowed into Hinduism from Buddhism.
Later on Buddha was given a place in Hindu pantheon as a reincarnation of Vishnu.
Another reason of Buddhist decline in India and in Central asia was islamic invasion. Though Muslims were equally brutal on Hinduism and Buddhism, there were some key difference between Hinduism and Buddhism. First was that Priestly class of Buddhists was very thin compared to Hindus. When Islamic invaders invaded and massacred Buddhist monks en-masse, they destroyed Buddhism as there was no one left to propogate Buddhism. They did same to Hindu Priests but the difference here was that in Hinduism, every Brahmin is a potential priest.To wipe out Hinduism, Islam needed to commit a through genocide, something which they were not capable of. Second is philosophy of non-violence of Buddhism. Buddhism completely forbids aggressive war , no ifs or buts. This made Buddhism weak militarily and was not able to put up as much a resistance to Islamic invaders. Compared to this, an Islamic invader had two choice in dealing with Hindus, either spend whole life on battlefield, or make treaties with HIndu kingdoms. This meant that Hinduism was less suppressed than Buddhism (Buddhism from Bihar was wiped out by Islamic Turks).This was primary reason of collapse of Buddhism in central Asia.