I think this is becuase Tyranitar has very good bst, and a good movepool and is very hard to take down with the big Sp.Def boost in the sandstorm he summons.
Also now when he's dark, this gives fighting types/moves a chance to counter him, also what pokemaster said, Tyranitar doesn't appear very friendly in the pokedex entries either.
I think pretty much the same thing of Volcarona, 130+ Sp.atk and 100+ in speed and Sp.Def, in addition to that, Volcarona also get the move Quiver dance, wich boost all those already very high stats with 50% each, and this is only 1 Quiver Dance, in addition to that, he also get stab Fiery Dance wich has a big chance to boost up his already massive Sp.Atk after Quiver Dance.
How are we supposed to counter this thing? rock types, he cleary flies/levitates and yet still hes a bug and fire type, i think because it's just fair that at least 1 type can counter him.
Sorry if i got a little of topic, but i hope this helps.
Don't vote me down because you like these to pokemon.