The reason has nothing to do with games. Pretty much, all games are available on all platforms.
The reason is Blue-ray. Is Blue-ray the best technology? No. Is it the cheapest? No. Is the PS3 a cheaper and better Blue-ray player than stand alone players? Yes. Not only is it the cheaper or the same price as stand alone players, but it is easily upgradeable as new Blue-ray standards come about.
The reason that Sony is brilliant is that they concentrated on incentives to the film distributors rather than low price. By securing content via incentives, they crushed HD-DVD, and no price cuts on Toshiba players are going to salvage HD-DVD. Game over, nice try.
This also means that over the next year or two, people will be buying a PS3 as their HD content player. There is a perfect storm brewing as analog TV signals are phased out, people are upgrading to HD TV's, and at the same time picking up a Blue-ray player.
Oh yea, that Blue-ray player also plays some games....
[Update] Fixed an auto spell correct error.