divers generally go where there reefs.  I've been to Goa and also Trivandrum.
The sub-continent of India, to the best of my knowledge, has no reefs and
access to the Laccadives or Lacashweeps located off India or whatever they're
called as well as Andaman and Nicobar Island off Burma require special visas
which has the effect of discouraging travel and access in terms of air travel
is limited.

The Indian gov't does not seem too focused on the prospects of diving tourism
which doesn't help matters.

Another factor which discourages divers to some extent in the Indian Ocean is
the limited time-frames for diving when one or other of the two monsoons are
not raging.  March and September are thought to be the best times.  I once
visited the Seychelles in July which was a bummer with the monsoon going full
blast.