Because if Ness' Fsmash was good, then his ground game would be ungodly. SO MANY DTILT FSMASH's!!! OHMG!!!
Reasons why Ness isn't too good in this game:
He doesn't have reliable strings. He has good move-strings, but most characters have either the range or the speed to prevent that string (also DI)
He's not fast enough. He can't rush after someone he pushes out.
Too little room for error. The fact that has short range (only fair is really big, UAir is big but not left-to-right, which is where ness is lacking), can be grab-released by most of the cast for safe damage, and has odd lag with half of his moves make it difficult to keep going in.
He's got a lot of potential, but a couple of minor things just keep him from being above average.