Would also like to add as somebody who plays both RU and NU, but the speed tiers in RU are a lot higher, Pokemon such as Mienshao, Gengar, Talonflame, Noivern etc are all prominent Pokemon there, and thus Lucario needs to utilise Tera Normal Espeed a lot more, which on its own isn't too super impactful. Combine that with a lot more base 100's like Entei and Jirachi that can stomach +2 espeed even, and you cannot really effectively smash through teams as much as you can do.
Adding onto the defensive counterplay part as well, for RU standards 110 base attack is not especially impressive, and thus Lucario typically requires a Life Orb to actually do solid damage. Here are some calcs to showcase this:
+2 252 Atk Tera Normal Lucario Extreme Speed vs. 0 HP / 4 Def Entei: 265-313 (71.4 - 84.3%) -- guaranteed 2HKO
+2 252 Atk Lucario Crunch vs. 252 HP / 252+ Def Slowbro: 210-248 (53.2 - 62.9%) -- guaranteed 2HKO
+2 252 Atk Lucario Close Combat vs. 252 HP / 204+ Def Chesnaught: 226-267 (59.4 - 70.2%) -- guaranteed 2HKO
This is pretty bad damage all things considered, and thus, you have to run LO, which heavily cuts into your longevity and makes you a lot easier to revenge kill by faster mons once you have been chipped down.
For NU standards, 110 base attack is pretty fine all things considered, the defensive walls are a good bit worse and the offensive Pokemon are slower and frailer and thus you don't have to rely on Tera or even clicking Espeed as much. When stuff like Slowbro was NU, Lucario was contentious but ultimately balanced. When they rose, there simply wasn't as many walls to it before and thus it got banned.