In French, apples are "pommes de terre." "Pomme" used to be a more generic word. Like how in England "corn" secretly means "grain" and thus "corn pancakes" could have oatmeal, or corn meal, or wheat meal, etc.

My bet is something like "apples have been poor man's food. Potatos became poor people's food, so we na;me it liek that" or something.