Let's be simplified about it then.
4 special moves + 3 tilts + jab + rapid jab + 5 aerials + grab + pummel + 4 throws + final smash = 21 seperate moves to decide.
Then you have to factor in how to balance those moves, how to make their animations flow, what to reference, how to make them stand out, a ridiculous amount of bug testing and how to make the chracter enjoyable to play.
Seriously, there nearly 80 characters, and for each new one you have to bug test how those moves interact with every other move.
And that's not factoring taunts, idle animations, victory poses, model development and everything else.