An important attribute of an e-sport is that it should be entertaining to watch. Since LoL is a game where a player controls a single unit, it's easy to follow along for the spectator.
I certainly want to also enjoy playing it, just watching it is not going to be very entertaining to me.
It's a bit different for me as I used to play LoL (stopped 1-2 years ago, ~4-5k games played), but now I only watch the championship and some league games (usually those my favorite team, Fnatic, are involved) without playing the game at all.