I imagine another major motivation behind the Xbox 360 being the lead development platform for many developers has to do with there being 8 million or so more Xbox 360's in consumer hands.
The 360 was 8 million ahead just this year. Now it's only 5.7 million all territories combined. The PS3 reaching the same amount of sales as the 360 by next year isn't so much of a long shot after all.

Now in the US only is 8 million ahead with the 360, but games don't sell in US only do they. In both Europe and Japan the PS3 is ahead of the 360.