Making the game go full multi-platform at this stage when there are already ME1 and ME2 on the market could hurt its future sales. I don't think that EA will take the risk especially when they see that their new game Dragon Age that is from the beginning a full multi-platform is selling much worse on PS3 than X360.
I think that they are able to achieve more when the game is a console exclusive for X360. Much more hype for the game.