because of the influx of new players at 1200, many of whom belong much lower, elo hell doesn't really stop until 1250 (at which time it gets less and less likely you'll be paired with an untried player).
This is only a problem because its a team game (so your win-loss performance is going to be dependent on other players performance in game), and because matchmaking doesn't take into consideration ranked 'experience' (trying to match players above a certain threshhold of games, ie 'proven' at their rank, with other similarly proven players over players with likely unstable rankings). In a game like chess, the influx of bad 1200 players doesn't matter, because they just lose and drop to their real elo, and don't drag anyone else down with them.
Now, some will get carried up, and i've seen players in the 1300-1400+ range who didn't deserve to be there. But the higher you go, the less severe their flaws get (its harder to get carried the worse you are), and sometimes their flaws are strategic - ie, they don't understand team composition, so some games they'll be playing the right thing and some games they won't. In the former they'll be fine, but in the latter they'll just ruin the game for the whole team.
Average game elo ~1380-1400
Players 1+2 pick fiddle and lux. Maybe not great round 1+2 picks, but not awful. (Fiddle ends up jungling)
Player 3 picks vlad (losing pick right there)
Player 4 (me) goes 'we need a tank', picks galio (best unbanned tank remaining, since bans were 4 tanks)
Player 5 picks Anivia (and that seals it, 4 ap heroes ftl)
Players 3 and 5 have no business being at that elo. They don't understand how to build a good team comp. I don't care how good they are in game, they lost the game for the team at champion select.
A related problem is people who play ranked and can't play every role. If you can't play any tank, you shouldn't play ranked. You're a liability to your team. If you can't play a jungler, you shouldn't play ranked. You're a liability to your team. (Generally finding players for other roles isn't a problem, but you should be able to play support, magic nuke, and ranged dps as well). These players might get to a decently high elo, and then fail their team by not being able to fill a needed role. Sometimes you win those games, but I don't need to tell any competent player how hard it is to win without a tank.
(ELO is implemented poorly btw. Currently a team gains or loses the same amount of ELO across all members. A real team ELO system would compare players ELO to opposing team's average ELO, and determine gain or loss individually).