Funding I suppose.
If you wanted to make a video game, & you lack the necessary funding to acheived the vision you wanted, or your parent company do not want to fund it. Someone come over & say, I fund you for your project, for a cut of profit or making it excutives, will you not at least consider.
More sales is a valid argument to release on as many platform as possible, but sales are the 'ending/result' part of development, & Funding is the 'beginning/drive force' of the development.
A game that doesn't get funded, doesn't get made. Its a very simplify reasoning for exclusivity, & I think you get my point.
Maybe Square-enix do not want to fund the next TR, or give them a lower budget, or worst, they want to cut the number of stuff to reduce cost. MS come in & made a deal, giving the extra funding needed, so they can get the game going, or justify the having all the stuff around. Its all theroy of coiurse, & I am not here to make MS the good guys, but TR being a major francise, unless there is something major, I cannot see how tehy will not want to keep it Multiplatform. Actually teh more I think about it, the more I thimk the stuff retraining part of my theory make sense.