Because the market accepted a subscription fee. Sure the platform providers have costs but so do Valve they just don't chose to charge for their services.
I guess if you want to pin it on one company then MS was first to implement it. That said the real reason it's now on all three main platforms (or soon will be) is because the market accepted it.
PC market refusing to pay and MS backdown shows it could have been rejected but it wasn't.
Whether it really makes much difference to the service I have no idea. Valve seems able to offer their services without one and the quality is very, very good so I suspect not but can't say for sure.
I guess if you want to pin it on one company then MS was first to implement it. That said the real reason it's now on all three main platforms (or soon will be) is because the market accepted it.
PC market refusing to pay and MS backdown shows it could have been rejected but it wasn't.
Whether it really makes much difference to the service I have no idea. Valve seems able to offer their services without one and the quality is very, very good so I suspect not but can't say for sure.