You're right, you can't just take Street Fighter, a game that was built and balanced around its current control scheme, and slap Smash's simpler control scheme on it. But there's no reason a traditional fighter couldn't work perfectly fine with Smash style inputs if it were designed from the ground up around Smash style inputs.
For me, it's two reasons:
1) It looks stupid. A character's just standing there, and then they're suddenly sliding across the stage. It looks absolutely ridiculous.
2) I like to play competitively, but I just frankly can't move my fingers fast enough to utilize wavedashing. Maybe this is a selfish reason, but I don't really care. Admittedly, I'd probably get my ass kicked anyway with or without wavedashing because I'm not very good, but I'd rather get my ass kicked knowing that I'm utilizing the same toolset as the other person instead of knowing that they have a tool that they can use and I can't.