Developers are not making games to make the prettiest game ever, they are in it to make money. As a developer, you want to make a game that can be sold to as many customers as possible. It doesn't matter if the PS3 was 3 times more powerful, (which it isn't), people won't buy a system with no games, and developers won't make a game for a system with few potential customers.

A new console needs to use it's first few months to build a decent userbase, Sony has not succeeded in doing that, so there is little incentive for a developer to spend heavily on a PS3 game. This is why so many of the PS3 games are also on the 360. From a developers point of view, you have the Xbox 360 which is far easier to develop for and has a good bit of potential customers, while the PS3 is complex, and not nearly as many potential customers. The PS3 is simply a bigger risk for a developer, and few developers can afford that risk so they don't spend heavily on development for it.