In my opinion UU is also a great pvp tier like OU and it is more than frustating to see that UU gets no love.
I think the most players play OU cuz they see just strong poke like dragonite, ttyar and so on.
Or the people see their fav poke in OU and automatically UU/NU are dead for them.
This is how i see this topic.
To improve the motivation of people to play UU/NU, they need something what makes UU/NU more attractive like @TheBeldumMaster (rewards) said.
Until now i have more than 3.5k hours of playtime and i participated at/in 4 UU ranked matches. (I`m sure that i missed a lot of UU ranked matches...)