I almost feel like it's less about the power of the smash attacks (or other finishers) and more about the fact that the faster characters tend to be able to combo into those as finishers.
Fox's up smash is great, but not solely for it's raw power. Rather, it's the fact that Fox has numerous setups for his up smash in various matchups that allow it to be so devastating (along with the fact that it is crazy powerful).
Contrast this with a character like Roy. Roy's fsmash is an extremely powerful move and would make for a very powerful finisher, however, Roy has limited combo options and especially has trouble comboing into his powerful moves like fsmash. Keeping it on the topic of up smashes, look at a character like Pichu: Pichu's up smash has plenty of kill power, but there aren't very many setups to land it as a finisher (certainly nothing along the lines of Fox's jab -> upsmash or Fox's waveshine -> upsmash).
Aside from smash attacks, look at aerial finishers like Marth's dair. The reason this move is so terrifying is because of Marth's ability to combo into it (as well as the almost guaranteed kill for landing it). A similar thing can be said about Falcon's knee.
To summarize, the quality of a finisher doesn't necessarily come from it's raw power (though it is an undeniable factor) so much as it comes from the ability to combo into the move or find other setups to land it. Typically, the faster characters are going to be able to combo into these moves better.
There are some exceptions to this trend if you look at characters like Jiggs, who despite being slower has good setups (on some characters) for the extremely powerful rest. As a rule though, the slower characters tend to emphasize gimps and other tools.
I really don't think the developers thought that the combo game of characters like Fox would evolve in such a way that they would be able to use such devastating moves reliably. Things like waveshines, the ken combo, and Falcon's throws into a knee probably weren't known during development (and aren't exactly something you would see in the casual play the game was intended for).
This is my perspective on things.