Alliance premades allow us to find 40 others who want to pvp and work to win. See in your 40man pug you have 35 or so players who want to crush ally’s and win.
Alliance pugs aren’t composed of the same type of enthusiasm. Out of 40 you’re lucky to get 10 try hards with the remaining 30 being trolls, afk bots, or fishermen
Alliance simply don’t have the same numbers of pvp players. Premades allow those 10 try hards a way to find and group with 30 or so other try hards.
The one advantage we have is we are on discord. But that really balances out with unfair horde racials and map advantages.
Also, horde turtle play style really requires no coordination or skill so even if you were all on discord it wouldn’t change much