The real question is why didn't Respawn include a multiplatform clause or whatever on their contract with EA? Did they just assume EA wouldn't sell their game to Microsoft?
Probably, yes. Exclusive deals on games being developed with PC as the lead platform are pretty uncommon these days. They most likely assumed that EA would want the game on as many platforms as possible to generate the maximum profit, just like basically everyone else assumes.
The question is what/how much Microsoft actually offered them. Not knowing that, we can't really say with any authority whether EA did something profoundly stupid, or whether they were made such an amazing offer they would have been stupid not to take it.