There are many reasons, for arena-oriented players however, WOTLK in the case of MOST classes offered a higher skill-cap than any previous or later expansion. This essentially meant more power in the hands of the player to both grow in skill and control their own destiny on the ladder, to not feel quite so restricted by meta and composition. Not that these things didn't exist at all, but you could go against the grain, while feeling like there was plenty of room to grow and continue to improve. It's relatively easy to master most classes in Legion, which holds a lot of good players back in a way. The recipe for success in Legion is just, have partners to queue with consistently who are capable of breathing without machine-assistance, and play a top tier comp.
Wrath PvP was more complex and that complexity allowed for there to be a wider bell curve or spread in how player skill manifested itself. It also just plain made it more fun for anyone who doesn't shy away from competition. It has/had it's problems with the S8 PvE gear and whatnot, but even that is worth the trade-off over scripted/pruned WoD or ultra-pruned Legion. Another thing to note that is Cataclysm began a trend towards homogenization between classes. All healers having dispels, all melee having kicks etc. this contributed to the degradation of PvP as well in my opinion.