Windows Store subscription is literally a buck a month atm, so your just kicking yourself in the nuts to support Steam imho. I'm sure Gabe's happy.
It's not about supporting steam, it's about not supporting platforms I don't like. And a buck a month? What is this? I have to pay them for the privilege of spending my money with them? Not trying to be a jackass I seriously don't know what their pay model looks like.