Why do they advertise the game only for xbox? and not ps3 or PC??

It's called pay outs. Bethesda is 100% in the pocket of Microsoft for this game, I'd bet Microsoft footed some or all of the development bill as well. Honestly though they've never had an advertisemant for other platforms. All the commercials say "Get your copy for the Xbox 360" at the end.

But, PC would be the superior platform. and 90% of people use microsoft windows.

Yeah but the game was built for the Xbox 360, so obviously that is the version they want to push, because Microsoft paid them to do so.

Quite a coincidence that their PS3 versions always turn out to be the bad version.

....maybe.