Why is that? The Pallava script is from the South Indian Pallava dynasty in 600CEish. All major SE Asian scripts descend from it.
Yet, the North Indian Gupta empire seemed to have far greater influence, and their script is Devanagari. Yet that did not spread to SE Asia. Why?
I have made a mistake in the post..i meant to write Devanagari, not Sanskrit
r/linguistics would probably give you a better answer.