To elaborate a bit, as sumwun said, Sky Uppercut can't be used in Gen 8, so that's a start.
This greatly hinders its usage, even though it was already a very mediocre move. Why? Well, really, the only Pokemon that flies is Gyarados (Bounce), and it basically only uses Bounce in Ubers where it gets the Max Airstream speed boost to pair with Moxie. This means that Sky Uppercut is only "useful" in NatDex. It's still not good though, because Gyarados a) will basically never use Bounce if its not Max Airsteam b) resists Sky Uppercut anyway, so it's easier to switch to a resist than get a little bit of chip damage.
Oh, yeah, and it's highly outclassed by Close Combat.
Hope this helps! :)