To some degree it's because certain options work better in other game modes and settings. For example, there are characters and karts in Mario Kart Wii which are only viable on a handful of tracks, but can dominate those tracks in time trials. A very basic opening cup track with only gentle turns might get dominated by a high speed character and kart combo with middling stats everywhere else.
That's why certain tracks were played with the Torpedo/Spear in that game. It's very fast, but has the turning radius of a bus on ice. Great on something like Mario Circuit, probably a complete catastrophe on something like Bowser's Castle or Rainbow Road.
There's also battle mode, but I have no idea which karts are preferred there.
> It's kinda weird that you can shoot yourself in the foot so much when picking your kart in this game.
Nintendo optimises for fun, not competitive play. Infamously, to curb the competitiveness of Super Smash Bros (SSB) Melee, they introduce a random tripping mechanic in the following SSB (Brawl). Everyone hated it so they removed it in the one after that.
I switched him to the meta build and suddenly boom, he went from like 10th to 2nd against the AIs.
It's kinda weird that you can shoot yourself in the foot so much when picking your kart in this game.