Simple answer, money.
I imagine the likes of Grays, Kookaburra, GM etc won't do a deal with a player without at least some sort of exclusivity, bat, pads gloves as a minimum.
Yuvraj is the most notable Indian who has a head to toe deal, with Puma. I suspect the rest of them look to eek extra cash out of multiple sponsorship deals, they won't necessarily get loads for using just a set of BAS gloves, but from promotional appearances etc.
The bigger brands who are happy to do it are just as 'guilty' (not the right word, there is nothing wrong with it in my opinion, but it doesn't seem to fit the commercial model most companies look to follow) for allowing these deals to happen.