I am also sure that Sony had to pay a pretty penny to get Mass Effect and Bioshock to go multiplatform.
Not sure if this is fishing or not, but in case it isn't I'll give a straightforward answer. Both of these games were timed exclusives. Microsoft was the one the opened up its wallet to keep these games as exclusives on its console for a limited amount of time. Once that period expired, it made economic sense for 2K and Bioware to port their games over to the PS3 due to the size of its install base.
That's why you very rarely see third party exclusives anymore. It doesn't make economic sense for a developer to sink a bunch of up-front production costs into a game for only one console, when you can almost double your sales by investing in a port to the other one too.