Due to Apple's restrictions, games that rely on Steam for multiplayer (say Borderlands 2, for example) must have their multiplayer elements rewritten in order to accommodate Game Center.
Serious problems arise when you consider that the majority of gamers playing these titles will be doing so on Windows, usually using a game service like Steam.
Right now that's Game Center, the not-so-mature matchmaking service you've probably noticed on your iPhone or iPad.
From a multiplayer standpoint, this restricts developers from using shared, cross-platform APIs and instead forces them to use Apple's solution instead.
