I'm not sure that we can rely on worldwide sales totals to tell the whole story. We all consider the XBox to have been much more successful than the Gamecube, but it only sold a few million more units.
Sales by region matter. Different regions have different tastes, and a developer needs to decide how much he ought to satisfy each region. A game that appeals to Japan is going to sell well in Japan and poorly in the US. A game that appeals to both is going to sell somewhat well in both, though clearly not as well in either as a more specialized game.
By concentrating on the American market, the XBox and 360 make it possible for devs to make games exclusively for Americans. A higher percentage of American console owners buy these games than would be expected, increasing the attach rate of the 360. At least, that's my theory.
The PS3's real problem right now is there's no reason to make a game for it. If you want to appeal to Japan, make it on the Wii. If you want to appeal to Europe or America, make it on the 360. If you're going for a worldwide audience, then put it out on the Wii in a few months.
And, as people have been saying, the 360 launched a year ago. Furthermore, PS3 sales are very bottom-heavy. It's matching the 360's track record only because it launched well - its future sales prospects are pretty dismal. I'd expect the 360 to continue to pull ahead by ~4m a year even if the sales rates don't change, and history informs us that leading consoles almost always increase their sales rates relative to trailing consoles.
