I'm not sure WHY unless you just really, really dig on the dual shock or something. I mean, most games developed using the 360 as their base platform end up cruddier on the PS3, and since it's apparently easier to develop for the 360 than the PS3... It's not rocket science or anything, is all I'm saying.
-My X360 freezes in games more often and causes me to redo sections which is annoying (my Lost Planet experience was basically ruined by this; Star Ocean 4 had a BAD freeze which almost caused me to quit playing, etc...)
-My X360 dies on me if I use it too much (I don't have a fancy falcon or whatever the new models are called. Launch-era still)
-My brother only has a PS3 for blu-ray and I lend him games I think he'd like (he liked and beat Psychonauts; he's not a gamer)
If I had a newer X360 that I could rely on and never think twice about, I'd probably get more stuff on X360. But as it is, I only feel that comfort when playing things on PS3.
Those are the main reasons. Then little things like most of my friends are on PSN for comparing trophies, the fact I enjoy the dual shock more except in FPS games (which I get on X360), the save game manager/XMB, the HDMI black levels look better than my X360's component, etc... are just minor things that add to it. If a game is significantly better looking on X360 I'll get it there (like in RE5's case I bought both X360 & PS3 version since I didn't want the graphically inferior PS3 one, but I wanted my brother to play it), but otherwise I'll just get everything multi on PS3 at this point. Though most the bargain bin X360 versions are $5-15 less than the same game in the bin on PS3; so I get X360 versions there.