why aren't there leaderboards for the Grand Prix mode on easy and normal?

Hard mode contains Maximum GP, a GP exclusive to this difficulty that appears after the race on Airport Oval.

Technically, the only way to 100% the game is to play it on hard, so it makes sense to only record Grand Prix completions on hard since Maximum GP is the “true” ending to the game.

Plus, hard mode makes the run more interesting since the races are more challenging, especially in the Ultimate GP. Easy and normal don’t require as much skill to win and are shorter.

sorry it took me so long to respond but you're right i guess i gotta figure out a way to win the last race then :)