The average amount of games bought per month of console ownership isn't that different between 360 and PS3. That's IMO the main factor.
Some other factors IMO are (1) Some games have been released much earlier on the 360 than on the PS3, (2) Microsoft released sequels to their major XBox titles relatively early and (3) some PS3 owners may not be that vivid gamers due to the PS3 doubling as an amazing multi-media device (much more so than the 360).
Also a lot of FPS/3D shooter games have hit the market, which traditionally sell relatively well on XBox consoles. (With upcoming games like Resistance 2, Killzone 2 and existing unique online shooters like Warhawk I think the PS3 will become a very important console for 3D shooter fans as well.) Especially considering online play is "free" on the PS3 if you already have an internet connection.
For the long run piracy issues (although RRoD may put fear in the minds of pirates), disc scratching issues (short term positivie effect on software sales, but IMO likely a long term disadvantage), RRoD issues after warranty coverage, etc I think may well push things into the PS3's favour.
