It's been many years since people went Horde "because the top guilds are there". At this point, people go Horde because they have to. Alliance simply doesn't have a large enough population if PvE raiding is what you want to do. It's a domino effect that has been going on for 10+ years at this point. For every Alliance player that throws in the towel and goes Horde, the recruitment situation is made even worse for the remaining Ally players.
It started with the top guilds during WotLK, Cata and MoP. And why exactly do you think the topguilds all went Horde during this period? Oh, who knows, maaaybe it had something to do with Horde racials being objectively superior during this era?
Prenerf Berserking while snapshotting was still in the game, was some of the most broken shit ever. Horde casters basically had another major CD that Ally casters didn't have access to. If Warrior 1 can use Recklessness, but Warrior 2 has to remove it from his bars, Warrior 1 will probably do more dmg, assuming equal skill. That's how Troll casters vs non-Troll casters performed in Cata and Mop. It was already OP in WotLK, when it gave 20% attack/casting speed. It's still crazy how Blizzard didn't see that making it 20% Haste would make it completely broken, when we were going into an expansion that were going to make all DoTs scale with Haste by default.