Small part because it's Xbox exclusive (admit it, some small part of the excitement for some games stems from the fact that a competitor won't get it.)
Much larger part because it's a fun, lively take on next-gen, where it's been a lot of military shooters and gritty realism so far. It's a breath of fresh air, game wise.
Yea...I mean, it couldn't just be because, you know...it looks fun? No, can't be that.
And I don't think anyone is realistically calling this a "system seller," (ok, sure people are saying that, but it's mostly hyperbolic system wars nonsense). I think this game looks phenomenal, and there have people been saying in the various SSO threads that it's making them want to get an XB1, but it'll realistically not be something that pulls "casuals" in by itself. But, together with many other great games that the XB1 is set to get, this game makes a great addition (and one that should be at the forefront of advertising the system, in my opinion).
And if it does turn out to be a system seller, that would be great, too (I hope it does). But, lets keep the system wars crap out of here. The game looks fun. It's pretty simple, really.