Because as long as this game is free to play, smurfs will always exist and there is nothing to do to stop it. Banning IP addresses doesn't do anything, if you really want, I can go into detail as to why. As long as you have a clean e-mail address, which let's be honest, isn't that hard to do, then there is nothing Riot can do to stop them.
In South Korea, you have to get a gaming licence, and there I believe you can make a max of 5 accounts on League, but even then that is only a slight deterrent. Most kids will use their parents SSN to acquire gaming licences, or even grandparents. Or if people don't like a certain game for example, I think HotS is extremely boring even though I have about 10 hours put into it, I could sell my account to another person/friend and they would have access to 10 other accounts.
TLDR: There really isn't anything we can do to stop smurfing.