Mainly because I didn't have a gaming PC built and games like Bioshock and Orange Box were better on the 360 than the PS3. Bioshock wasn't even on the PS3 at the time (Fall 2007). Also for Gears of War. Bought Halo 3 when it came out and then felt pretty stupid about buying into the hype when it turned out to be essentially the same game I didn't like in Halo CE and Halo 2.
And thanks to the HD-DVD/Blu-Ray format war, studios were split down the middle on which format to distribute on until HD-DVD cried uncle and quit. If you really liked movies, one format wouldn't have all your favorites before Blu-Ray won.
In retrospect, it wouldn't have killed me to skip the 360 had I built a game PC first. Now it's just an extra option to play on.