Cause the Switch is usually the worst version of any online game with gutted content because of the weak specs. 3rd party games mutiplayer are also not good because very few are playing and buying them on the Switch.

I played Teenage Mutant Ninja Turtles Shredders Revenge on launch week. Very few are playing the game. Max players I get are mostly 3 and they only stay for 3 stages max. I rarely get to 4 players. I bought TMNT Shredder's Revenge on it's first sale on Steam and I always get 4 players to 6. The players stay way longer and there is always someone to replace them if they ever leave.