This is the main reason, IMHO.
For many years Vray was only available for Max, and Vray turned out to be perhaps the best rendering engine, apart form proprietary ones or RMAN.
Now that Vray is also available for Maya, one can use whatever he likes, although I have started having thoughts about switching to Maya: the reason is that Maya is better integrated with Nuke, which is another must-use recently. I can foresee Maya-Vray-Nuke workflows will be the standard thing in a couple of years.
My 2 cents,
Tassos Ringas