Riot openly admitted that they play favorites back when the forums were around. It was back when many were wondering why Lee Sin and Riven never received real nerfs while some champs got completely gutted. IIRC their argument was that not only are these champs among the most popular in the game, but many pros enjoy playing these champions so they never get hit too hard by changes, or at least not for long before receiving compensation buffs/skins.
Kind of sucks for those that main the lesser played champions, as it makes them feel like second class citizens when it comes to getting any kind of positive changes.