There seem to be a limited number of u-row characters that do not have verbs associated with them, and I'm curious why some of them do not exist. There once were ふ verbs, but there are no ぷ、ず、ゆ、づ verbs. I might as well throw ぬ in there because there are only one modern and two older ぬ verbs, so, while not nonexistant, that's pretty rare.
I'm curious if there aren't more ぬ or any ず verbs due to those being formal conjugation endings, though the others are not conjugations, so that wouldn't explain those.
Though, I'm specifically asking why these no longer exist. Even if ずる verbs changed over time and disappeared, there's no reason that new ones ending in ず couldn't come up.
Also i've never seen an answer for why there are no ゆ verbs.