Well, you figure that Sony released some of the PS2's biggest and most impressive games -- God of War 2 being the best example -- something like several months after the PS3 had came out. For all intents and purposes, there wasn't really a *need* to upgrade. The people who did were the die-hards that always have to get the newest console at or shortly after launch; the price tag didn't help much either.
As for the Wii and the 360, their previous platforms had basically lost to the PS2 so the next console cycle was a logical move. I think that MS has done awesome stuff with the 360 and we're just now really starting to see it shine. And the Wii, well, Nintendo isn't trying to capture the same audience that MS and Sony are going for.
That's just my two cents though.