While it is in some sense just a remake of the original game, it is still a fairly high-profile product. I could see announcing that it would be on Steam maybe a year after it had been published, but to announce it will be on Steam at this time probably means you will be able to buy it on Steam the same day it's available in brick-and-mortar stores. That's big news.
It is also interesting that Eidos has previous cut a deal with Steam's biggest competitor, GameTap. Tomb Raiders 3, 4, 5, and 6 are all available through that service and Lara has been featured in GameTap advertisements. Here's one that they're currently running on their website:
It would appear that Eidos is testing the waters with more than one online game delivery service. I wonder which will look more attractive to them in a year: the ongoing residuals from GameTap or the outright sales* of Valve.
* Yes, I know it's not an outright sale: you still have to have yourself plugged into the Steam network. I still dislike both services intensely.
How does GameTap compare to Steam anyway? After my miserable experience with the latter, I haven't tried it.
And it's sad - I was just thinking as I'm boxing a lot of obsolete games for charity ... I'm just the kind of guy who would love these services. If they just were customer friendly.
In short, Steam is closer to a traditional buy-a-game-in-a-box model, but with a network tether. GameTap is an all-you-can-eat subscription model.
Steam: games stored on your system, many/most require network ping back to the Steam system occasionally, only play the games you've paid for, limited selection, no periodic fee to play games you've paid for.
GameTap: games exist on service's servers and downloaded on demand, requires network connection all the time, play any game on the service, large selection, regular subscription fee must be paid.
Another HUGE difference between the two is the availability... For me as a European, I am unable to use gametap. I do prefer Steam over going out and getting a copy from a store though, mostly because some European countries either translate games and/or censor the best parts out of the games!