Latest Tomb Raider sold 8 million copies, mostly on PS consoles (Definitive Edition's sales on PS4 vs Xbox One in particular was skewed heavily in favour of PS). Do you think Microsoft will manage to push even half of that on Xbox One alone? Not happening. And this decision was done long after it was clear PS4 sells more so it's really puzzling why Square did this
Probably because of funding. Apparently it took SE some time to break even with the TR reboot even though they sold over 1 million copies in the first 48 hours because the development costs were so high. So SE bets on getting funding from Microsoft which will mitigate their financial burden and risks, Microsoft bets on being able to boost the sales of their console by financing the development of games from popular franchises on the condition that they become timed exclusives or have exclusive material on the XBO, and both parties hope that luck will be on their side and that they'll be able to make a profit out of it.