Because it is completely Riot's fault for attracting racist players.
Untrue, Riot has made a fun and popularized game, with the player-base increasing more every day, there are bound to be racists.
And are people really complaining about LoL? This game is largely localized in North America, in comparison to DotA for instance the amount of racism is much higher.
Some of the "racist" ideals of players have their legitimate roots. I for one have played multiple games where players were of a different ethnicity/country and I could not communicate. I try to tell them to gank or offer advice on what items they're making, (since they're doing it wrong,) however none of my messages get across the language barrier. LoL like any MOBA is a heavily team-based game, and if the team can't communicate then it's potential is severely crippled. Although I have no personal problem with any of these players, shouldn't they be playing in servers based in their own country/area?
And in the end- should you really even care? Racism has been around practically since human civilization began. Do you honestly truly care what some random people think about you and your race? If you brutally destroy a player in a game and makes fun of your race, isn't it just that bad player trying to make up for his/her own insecurities in a pathetic and insolent way?
If some random player, who could easily be a jobless 40 year old living in his mom's basement, calls you a ****er, should you go cry about it? Or should you realize that you're clearly a person living up to a higher standard and care not about some random's person's worthless opinion. The main reason racism is still such a big deal is because people care too much- it's not a big deal.
I just read a thread moments ago by some Asian kid who saw people calling others ****ers and was "offended." I was like seriously dude?! You're not even black, and if you were, why should you care? If racism really offends you that much, why flood Riot's email inboxes with useless complaints to ban players who will simply make new accounts, why not just lock yourself in a cave and never come out? Because all societies contain racist people and racist remarks will continue to be made for a long time, so either laugh it off and accept it, or just suck it up and stop complaining.
tl;dr No one cares, stop complaining.