Yeap, that's the interesting thing about PC. Steam is big, but it's dominated by non-AAA games for the most part, things that are fairly smallish in size compared to major console releases. And of course there are still PC games even at WalMart on DVD due to the bandwidth problem that many face.
Steam / etc would be ubiquitous if the infrastructure could support it, but as it stands there are a certain number of people who are unreachable with it in a total manner.
There are a fairly large number of people in the uncomfortable 'mediocre' zone. What I mean by that is ~6mbit and under connections that even at their best aren't too hot, and a lot of families have to share these, so say someone with a typical garbage 1.5mbit line (I personally know of a heap of these exact people, even in nice new developments, and there is no option for better speed where they're at), yeah their kid will be in trouble trying to grab that Halo 5 @ 44GB. At best it will take many, many days, and that's if nobody else is using any bandwidth. A 1.5mbit line is barely enough to scratch along a decent Netflix film in sub-DVD quality.
These are the people that can use Steam + little games, or XBL + the games that they buy retail, but who can't DD anything full-scale.