Because moneyhatting is a dirty practice and should be frowned upon. I mean this isn't some lousy DLC that launches a few weeks earlier on one console (which is still questionable, but more forgivable.) This is a game franchise that has always been multiplat, a sequel to a multiplat, and a game that started out multiplat. Even worse, it's a middle finger to those that supported TR the most, PS fans.
It would be one thing if this was a new IP that MS picked up early and helped fund/develop, and was now exclusive (timed or not.) This is what they did with Sunset Overdrive, and what Sony did with No Man's Sky. Or if it was a sequel that never was going to be made and they stepped in to fund it, like what Nintendo did with Bayonetta. No, what this is is MS getting desperate and wanting to add something to their arsenal and to have something to compete with Uncharted 4. So, they'll just pay a crap load of money yo keep it away from gamers who don't have their console, when they would have, otherwise.
If this does turn out to be a timed exclusive, I hope PS4 gamers give the finger back and don't buy the game. Basically saying, "If your loyalty can be bought by anyone with a big enough checkbook, instead of being with your customers/fans, then Uncharted is good enough for us."
