Yet, other competitive fighting games that utilize P2P connections such as Street Fighter V, The Mortal Kombat Series, TEKKEN, Dragon Ball FighterZ, Soul Calibur 6 don't suffer from this issue...why is that?
They do though, if you played them enough - you would know.
Here are some examples.
I think the issue with Smash is its mass appeal, which means it is played by both hardcore and casual gamers. I imagine that there are a LOT of kids playing Smash, kids who know nothing about networking - using stock WiFi on a shared family network while their parents/siblings collectively rape their bandwidth. Until these people either improve their connection, or eventually leave the game... We're stuck with them.
Streamer/YouTuber Maximilian Dood actually paid for two broadband connections at home, so that his streams would not cause any problems with his gaming connection. Yes, he can't always avoid laggers. But if the people he connected to cared about their internet connection in the same, they'd be practically guaranteed to have a good time (providing the regional matchmaking does its job). So to think that there's streamers out there spreading "facts" about lag, while they run a 1080p stream, while streaming music and having whoever else using their connection...
Yeah, it speaks for itself.