The reason WoW never has been and never will be an e-sport is very simple actually.
WoW arena is not good PVP in any way, to watch or even to play.
It's entirely UI driven and has no early/mid/late game or interesting meta. The classes are very unbalanced, the gameplay is stilted and gear-driven - basically it's garbage.
The only people who care about WoW PVP are people who play WoW, and even then very few as most WoW players are casual PVE players.