it's makes a lot of sense. 2 save files would take up a an extra 5 mb of data and nintendo would have to develop a screen that allows people to pick which save file they are intending to play.Lazy devs don't exist. Just devs that stick to budgets.
while technically possible,.. that is just too much effort for a nintendo title.
It's frustrating because this isn't that though. The devs put in extra work to make this feature. It's a cool concept but not fleshed out enough to be forced as the only option.