A lot of people don't play ZSS because of her learning curve. Usually the counter argument is to look at Snake (who DOES have a high learning curve once you can't win with tilt spamming anymore, and actually need to use traps and setups), but even new people can win with him somewhat. Plus, how many characters do you know have a learning curve for recovery? lol
ZSS, Link, Ganon, Oli, Ivy, and Ness. Maybe Mario. Thats all I can think of. So yeah, thats another thing that might turn people away.
Also, this might sound stupid, but some people may not think of her because she's not even on the selection screen, so when you're scanning the screen for a new character, you don't see her. Plus she doesn't show up for Random.
All of these play a role, IMO.