While you wait for fresh answers by more qualified users, you may be interested in some past threads on the topic.

Why did Buddhism die in India but thrive in neighbouring Sri Lanka? with a detailed response by /u/huianxin, not to mention a FAQ-worthy list of related threads.

What exactly happened to Buddhism in India? How did it go from being an effective State Religion for nearly a millennium to an also-ran by the time of India's Muslim Invasions in the early 1100s? with a detailed response by /u/JimeDorje.

The Indian subcontinent was the birthplace of Buddhism. The teaching of Buddha found lots of success in foreign lands such as China, Myanmar, and Indochina. Yet in its homeland, it is currently a very small minority. How could this be? with a detailed response by /u/iitii