For raiding it was kinda just “Alliance is marginally better but nobody cares unless X mechanics makes you swap over because everyone is already horde because of how things have built up over the previous 7+ years”. Contrary to popular belief, players don’t actually care that much about min maxing dps down to the nth degree when it involves paying IRL money to race change.
It’s only recently that bliz have started making powerful otherwise unremovable dots core raid mechanics, hence the spike of dwarf.