Their system has always had a dynamic slant to the filtering. The longer it struggles to match against the current limits, the more it loosens those limits. When the available player pool thins out a lot, the matching limits tend to hit the floor, so you wind up with much wider ranges of skill levels and/or latency. And when a lot of us have stopped queuing up.... well, there is a large part of your answer.

The reason why it's not so easy is because you're expecting it to be a simple matter of 6 on one side and 6 on the other. Make sure they're both evenly matched teams and you're away. Fact is it's not that simple. Say that skill is ranked from 1 to 10: Team A is made up of the following players: 2, 4, 8, 1, 8, and 7 Team B is made up of the following players: 5, 5, 5, 5, 5, and 5 Is that a fair match? Both teams have the same combined skills, surely it's balanced? Will Team A's more spread out players balance out in the end? Will the 5's on Team B kill the 2's and 1 on Team A as consistently as they're being killed by the superior 7 and 8's? Or will the 7 and 8's steamroll the 5's, leaving them to be dead more often than alive and not having the chance to even find the 2's and 1? The obvious answer is to have 'pools' of players in the same match, say, only 1-3's, or only 7-10's. But then you run into a whole slew of other problems. Increased queuing times, increased latency, etc. Fact is, matchmaking isn't just a matter of 'balancing two teams of six'.