It happens because Steam creates competition. If you think MS would just magically drop prices if Steam or other digital distribution services (Including Sony PS but less so) weren't around you are sorely mistaken. It took them quite some time to even come around to price drops to begin with. Obviously this has more impact on multi platform games than exclusives.
Also, it's hard not to praise Steam when they pretty much started the pennies on the dollar games pricing wars. Now you have Amazon, and all the others jumping in, creating even more competition.
On topic, I am glad to see that they have made some changes to Forza. +1 for the whiners and entitled.