She's so straightforward that despite her drawbacks Riot isn't serious about her high level viability. Without high level visibility she gets a limited market of players and no screentime and little attention. A lot like Garen.
Yas/Zed/Lee can execute more mechanix displaying so much skill and fun
Olaf which is "the same champ" has intense enough micro to not be overly easy so he can be good when played well
Riot also has a hard on for utility champions and "dependent carries" because it creates a gameplay model for teamwurk. So while high mechanix assassins fit one bill they don't fit that one.
They also love ranged and may never fix the difficult problem of their laning advantage because they have more depth in positioning.
Shyvana doesn't make it into any of these camps.