TBC PvP Players and Wotlk PvP Players tend to have different interest of game-play. I know a ton of players from the TBC who has no interest or desire towards moving towards Wotlk PvP, and I understand why.
I myself am leaving WOTLK PvP for other games because getting destroyed by Bis Gears gets old and boring quick. The problem is almost everyone is fully geared and skilled, likely because many people not in Bis prob don't stick around long if they have no Path towards getting the gear. Blackrock is not a good place for casuals, it's the most hardcore PvP only realm I ever played on. Icecrown players which all seem to be fully geared frown upon new Blackrock players joining because they know players who start on Blackrock mostly suck.
Although the difference between the starting gear on BR and everyone else is MASSIVE, with Bis geared players melting people, that's not the major problem I have with WOTLK PvP.
When I played AV in TBC, it felt like an actual battle. I never had a game were one side was GY camping the other team. In Wotlk it's almost every game, and it's generally one side steamrolls the enemy team, swarm in, GY camp, take all the towers quickly, and win.
In WOTLK, Galv just gets nuked. In TBC, it's an actual battle that required strategy to kill. There were epic moments were a Off-Tank would pull the last tower guard to the side while the main tank worked on galv.
More Specifically, Classes are more balanced, Battles are longer, More Counterplay, no Ret Paladin button smashing, more strategy, and gear mattered less.
This is why TBC Players are not moving Wotlk PvP, were into quality PvP that feels more natural. Were also not playing here because the server is dead!
Overall, Wotlk is doing great with over 24,000 players on, but only 3 of 78 players I added to Discord are actually playing on WOTLK, most moved on to other stuff or are playing Classic. I feel that if TBC had good infrastructure like WOTLK, there would be 24,000 players on WOTLK, and 6,000 players on TBC since TBC players and WOTLK players generally don't play each others expansions.