After let's say 500 games, bad players are not going to be winning all of those games, which will keep their rating lower. Good players will only improve higher in the rating when they're grouped with other good players.
The problem is, even if you're a good player, you're still going to have an equal chance of getting queued with bad players or groups with no healers. This past saturday, I played about 20 games as a solo queue. I won 2 of them. That isn't because I'm bad like the rest of my team - I'm not. But when you're grouped with a bunch of people in recruit gear, mixed PvE greens, and no healers, even in full modded WH gear, I can't do enough to swing the balance. So, by your suggestion, I would have a crappy bottom level rating because of those 20 games instead of rising to the top where I should be.
Again, reducing the queue to solo only puts you at the mercy of the random group builder and at the mercy of random gear and skill level of your other 7 teammates. Hell, right now, you can't be guaranteed of having a full group let along being grouped with 7 people of equal gear and skill.