They have some tricks they can do, like route all general guests down the stairs after entering the land through Peach's Castle, leaving the whole left side upstairs accessed by only those participating in interactive experiences, in case the boss battle or inside activities queues spill outside.
Sure that means you'd have to go downstairs and then right back upstairs to ride Mario Kart, but I bet it helps Yoshi attract more riders as a speedbump, taking guests away from Mario Kart than usual.