I don't think they ever really knew what to do with Sulu.
Chekov was a much more interesting character. It seems like the writers all through the series and movies thought so too. That's the way it appeared to me anyway.
Except for the pink schoolboy outfit in STFS and the TMP "torrrrrpeeeeeeeedoooooooes ... awaaaaaaaay!" line, that is. That was just goofy.