The third party console exclusives. MS has published a lot of them to ensure they are only on Windows and Xbox. You think MS just writes a check so they don't put them on PS4? No.
First publisher rights doensn't means it will be only in one plataform... dozen of games published by Microsoft ended in others consoles... you need a contract of exclusivity to that and it can be in the publisher contract or not.
To have a contract of exclusivity you need to offer something to the dev... and that can be a lot of things like Marketing budget, cover dev costs, help to developer the game (which MS don't have any studio doing that) or even a $$$$ check.
That is how a game ends exclusive.
The issue is that if you sell less while the competition sells more (a lot more) the dev start to see that only a marketing or developer deal won't cover the amount of money comming from releasing in this other more successful plataform... even the check start to be bigger than before... if you paid $50m before this gen started now you will pay over $200m.
That is how it works.
This is business... and MS don't have the priority to get exclusives due the low sales... they need to offer a lot of money in one way or other.
PS. I don't have any issue against to buy exclusivities or time-exclusivities but for MS it is becoming more and more expensive and the results are not coming.