Why hasn't League of Legends been ported to Linux yet?
I can't really think of many explanations as to why Riot hasn't decided to port League of Legends to Linux. Most Linux distributions are far more stable than Windows, and a lot of them more stable than Mac. The Linux population is ever growing, and more and more games are being developed for Linux now.
Sure, porting a game like this to Linux might be difficult, but it'd be seriously worth it in the long run. I can guarantee this is something that won't be bad for any part of the community.