Leaderboards are allowed to allow or ban versions of the game or emulators as they see fit. If the emulator does not emulate the game well, and I recall that dolphin is not the best at accuracy since it favored playability, they are allowed to ban it if it gives unfair advantage over console. If you feel passionately about this, you can take it up with the mods for the specific leaderboard for the game you want to run.

In the case of Wii games, Dolphin is almost always banned for 2 reasons:

1. Much faster load times than console (which tbh varies a lot per console)
2. TASing in Dolphin in an undetectable manner is super easy. This video provides an example: (read the description)