Yeah, I'm going with money and the fact that microsoft seems more deadset than ever this gen to make it look weak by slowly chipping away at its exclusives. The fact that PS3 is pricier and generally lousy for console shooters compared to the 360 doesn't help either, nor does the fact that the 360 came out at a later date and had to play next-gen catchup.
There is a longstanding rumor that Microsoft purposely wants to remove Sony from the console race for various reasons, I believe the biggest being that Sony is one of Microsoft's biggest rivals for the king of electronics throne (don't quote me, I myself am quoting random groups of fanboys here). This would make sense given how many annoying deals microsoft has made lately that seems to screw over the PS3 owner and how one-sided advertising is. Two items in particular are the Resident Evil 5 ads I've seen that say, "only on the 360," which is obvious BS, and the other is the fact that The Last Remnant still hasn't got a release date for the PS3 version in spite of originally being announced to come out at the same time.
Of course many companies are milking exclusive titles these days. Star Ocean comes to mind, most of its games being on Sony consoles before becoming a 360 exclusive in its latest installment. I was never into the series but I'm sure that's upset fanboys somewhere. One series I was interested in was Crimson Sea, who's first title was X-Box only and who's sequel was PS2 only. And lets not forget the Tales series, which seems to jump console every generation. It's quite tiresome, really.
