It was never going to be an Xbox Exclusive for the life of Xbox One. I wouldn't be shocked if I found out Microsoft and Square cooked up this deal as a way to gain marketing for the game, bad press is great marketing and all that.
The game not coming to platforms that the IP was born on basically drums up attention and no doubt some people will consider it a reason to buy an Xbox One, along with Halo 5, Quantum Break and the next Forza over the Holidays, along with any new games exclusive or otherwise announced at E3 or GDC for the second half of 2015.
CD could have just waited until a few months after Uncharted 4 launched and give themselves more time to polish off Rise of the Tomb Raider on all systems, but it wouldn't have had the same kind of impact as this deal has in the gaming community, so it was kinda clever from that POV. Going exclusive does let devs put better focus into making the most of the systems they're launching on, maybe when this comes to PC, PS4 and PS3 it will be a more solid release because the devs have been able to optimize the title better and their engine across all systems.
