Why isn't there a 20 star full game leader board, like SM64 16 star?

There is. Click on "Misc." and it will appear.

I mean where you complete the full game, not just the first bowser.

Because the original 20 star category was a popular race category which ends at the first key and barely anyone did runs beyond that. Therefore I don't see why there should be a separate full-game leaderboard for 20 Star.