Why was the subcontinent less isolated from outside influence than east Asia?

Answer 1

Because it's more easily accessed from the Middle East via land and sea. Human migrations to-and-from the subcontinent through the middle east have taken place much more frequently than between the middle east and the orient.

This could partly be geographic, because despite the high mountain ranges and passes, it's still easier to get from the middle east to the subcontinent than to traverse the desert that sits between Central Asia and eastern china. Furthermore, as maritime trade became more important, it was easier for Arab and Indian traders to travel than for either group to travel, via the Straits of Malacca, up the South China Sea, to the ports of China. The monsoon winds only blew in the right direction for a few months in the year, meaning a round trip journey from India or the Middle East to China could easily take over a year, with a stay in S.E.Asia.

For all these reasons, and because of migrations of groups (such as the Aryans), there has historically been far greater cultural exchange between the middle east and the subcontinent. And through the middle east, there was also cultural diffusion and contact between Europe and South Asia.

Without even attempting to talk about all the foreign invasions of the subcontinent, from the time of Alexander the great, through the Persians, Mughals and Europeans, I think it's easy to understand why the subcontinent was less isolated from outside influence than East Asia.