Nobody is forcing them, they made that decision because it's the most profitable to them. And they definitely did not lose a few hundred thousand sales.
Look it up. They lost a lot of PC-Veterans due to Steam but they gained a lot of New-Players to the series. It is currently profitable due to the new people but to be honest, doing this with Steam will lose profits in the long run.
I also can easily see the # being in the 100,000s (Compare to the possible 10,000,000s of people who play Skyrim). From my math, this is what I calc'd doing simple math about the % of people who plays TES series and the % of people who has Dial-Up or No Internet at all.