well it also always was that the console with the most userbase gets games timed exclusive and then ported which helps the console alot for example ps2 with GTA

Cost is everything when it comes to game development....

There are two things that determine an exclusive versus a multiplatform

1) money, obviously this is teh biggest one. If you aren't totally sure that one platform alone will get you the return you need to make money off of the game, then you'll have to go multiplatform

2) the ability to port games, this one really only seems somewhat relevant, in that if you create a game like mass effect or a highly graphic intensive game, it's not going to show up on the Wii, but if you make something that isn't graphic intensive, but focuses on Wii's specific motion controls in order to get the most out of them, it's unlikely for that game to be worthwhile in porting it to the 360 or PS3