In the ROM, there is a line of text with eight character names: MARIO LUIGI PRINCESS YOSHI TOAD KOOPA DONKEY CASSARIN. The game's player select has pictures of the first seven characters in the same order. The eighth name, Cassarin (Catherine), as you may know, is Birdo's Japanese name. It appears that Birdo was cut from the game, which might explain the odd amount of seven playable characters.

No love for Birdo

Thanks: RaccoonSam