How to explain the diversity of cultures in South Asia?

There are several reasons for the diversity of cultures in South Asia. Firstly, this region was inhabited by several waves of people from different ethnic groups. First, neighboring Austronesian tribes penetrated here, then the Dravids, then the Indo-Europeans, Chinese, etc. Secondly, in this region, the first civilizations appeared in river valleys, and between it there were impenetrable mountains or jungles, so they existed for a long time in isolation from each other.