Fun fact, this game actually started from a conversation at lunch. It was never meant to happen, but ended up happening anyways.
I think they chose Xbox for 2 main reasons.
1.) The majority of the Xbox fanbase love shooters more so than the Playstation fanbase, and so shooters usually sell better on Xbox platforms. (just look at X1's 2 best selling games - Cod & Titanfall.
2.) Microsoft has a verry good online infastructure, and are offering thier Xbox Live servers for use to developers for verry reasonable prices. This was a big part of the reason why they went with Xbox as they want to incorperate some big online features with SO.
I don't think they are tring to make enimys with sony, rather, they are proving thier independance from sony.
