Zack is a complicated one. He was a Legomaniac at childhood, a Power Ranger at adolescence, did voice over for Broly, and was a part-time Teletubby before getting his big break with DOATEC. I'm too intimidated to use such a character.
Becuase mashing with him gets them nowhere. He actually takes skill, thats exactly why no one uses him. Same can apply for Akira, Pai, Leifang, Eliot etc etc
In fighting games people will tend to go to those that are popular. In reality it has nothing to do with them being hard. People use Hayabusa even though he is hard to use.
There is one person who uses Zack I know of even though he is a trash talker. A guy called Shikimaru.