Why does Classic WoW PvP feel so much different than Warmane?

I am playing a Destro lock on Classic, I have the same exact P1 BiS as my Destro Lock on Blackrock and on Blackrock with identical gear I can go into BGs and Arenas and perform pretty well and I see myself doing good.

In Classic even with the same exact setup, I feel like every other class is much more oppressive and it seems way more difficult to climb in arena.

I understand there is a seasonal difference, but even with same gear Blackrock just feels almost easier to PvP in.

Does Warlock just get better in Classic overtime and they're bad Season 5 in 2s/3s? Or is this is a different issue.