Then you possess an anomalous psychology. Typically people form a sense of the future based on past patterns. With two decades of TR games on multiplatforms, the expectation would be that it comes to multiple platforms in it's next version.
I am very surprised at other's surprise, but understand the disappointment.
Anybody who has been following or contributing to the
Xbox : What should MS do next? and/or the
What HW cost reduction options does XB1 have? threads will have deduced, based on what information we have available, that Microsoft don't have a lot of options for increasing the appeal of Xbox one. Their tech has less whizzbangs than the competition (and issued for some) and there doesn't look to be great scope in cost reducing the package without taking a hit on every console sold. For the most part, people want games to play on their games console and, in my view, Microsoft's first party stable is looking lean compared to their competitors.
I think third party exclusives is one of the few options Microsoft have that is achievable and this is why I'm surprised that others are so surprised. From where I sit, Microsoft buying exclusives was inevitable and I'd be astonished if this is the last third party exclusive Microsoft have bought or are in the process of acquiring.
So when folks say they didn't expect this, I can't argue with that, but what I would say that a lot of people haven't really been paying attention. I'm not disappointed a) because this didn't really surprise me and b) by the time it launches I'll have an Xbox One anyway
Like the little green dude said about fear leading to anger and anger leading to suffering, assumption leads to expectation, expectation leads to disappointment. Everybody who assumed [the game would be multi-platform] is now disappointed. I assumed no such thing - mostly because I really don't think that far ahead for games but I would say that Tomb Raider, which did well in 2013 and has seemingly done well with the Definitive Edition, is the perfect franchise for Microsoft to grab as an exclusive. The brand is doing well and PlayStation owners are likely to know it and crave it all the more.
Whether it'll actually sell Xbox One consoles of course, is another matter.