With the percentage of people who leave, its really a lottery and a matter of time to when you can break out of the low elo, high leaver games. Some people are just going to have bad luck and get stuck with leavers over and over again, sometimes they'll get stuck with leavers in games where they probably should have won, but lost because of the leaver. Sometimes you won't get any leavers but it will be in a game that RIOT thinks you're suppose to lose, and you'll lose anyways.
Due to these circumstances, its very well possible that someone who is good can be stuck in low elo indefinitely due to this bad luck.