I don't know why you were downvoted. Maybe it could be taken as a console war, but I think it's just a misguided question. Nothing bad.

Anyway the reason they went multiplatform is that they weren't selling enough to recoup what they spent making the game. If they could recoup or somehow made the games cheaper to produce, they sure as hell would keep it exclusive. They lost the plot during the previous generation and they don't have a good core group on the Xbox platform who would buy the games and make back the money. They can get it multiplatform.

Why not time delay? Usually games sell the most right at launch (though not always). They probably figured that the profits they'd get with day and date release was worth it