Is because she is the easiest champion in the game to play.
Aside from that, she has absolutely nothing going for her, and is actually a terrible champion, that will never be good, or comparable to Alistar, until Zenith Blade grants her CC immunity upon tagging a target.
The only reason her E was allowed to be garbage in the past, is because she actually did damage, and was actually a threat.
Her Q does 10damage now. She doesn't do damage anymore. She is not a threat. Give her E quality of life compensation buffs for neutering the only thing that was ever good about her(Her burst).
As it stands the only three support matchups that don't hard counter Leona are, Blitz, Soraka, Sona.