Oracle DBAs get paid more because Oracle is a pain in the ass to administer and the documentation for it sucks.
SQLSVR is much easier to administer, there is plenty of good information in Books Online, and hence there are plenty of SQLSVR DBAs. Supply and demand.
Of course, how many of those DBAs are GOOD at it is another matter, but practically anybody can do basic SQLSVR admin through the GUI.
I have found, however, that SQLSVR DBAs are better sql developers than Oracle DBAs, simply because Oracle DBAs have to devote so much effort into just running and tuning the engine.