Because traditionally players on different factions couldn’t interact, thus you’d lose your friends for a tiny community that would have no reason to grow.
Of course, after allowing players to play cross-faction regardless, this has all changed…
As far as I’m concerned, at this point all bets are off. Split the game into 8 friggin’ factions and make Warmode the hellscape is deserves to be
If you choose to see it this way you’ve not been paying attention for the last 20 years. The factions are not a idealistic monoliths. Each race has their own goals and motivations, and there are internal conflicts within the factions because of that.
The factions themselves are just reflections of their current leadership, but within them there are a plethora of different ideologies and convictions. Look at real world “factions” like the EU or NATO. They are alliances that benefit each member in some way, but those members don’t all agree with each other on everything.
The current Horde couldn’t be further from “Zug zug if you don’t join us you are our enemy , Dabu”. If you go back a decade when Garrosh was warchief, maybe, but not before him and not after.
Just look at the Tauren. They’re about as peaceful as they come unless you mess with them, and you don’t want to mess with the Tauren. Yet they’ve been part of the Horde since the beginning of WoW.
Factions are pretty meaningless in World of Warcraft, bar the few instances where it bars you from playing with your friends (I’m looking at you LFR/Normal, Heroic and TW Dungeons).
They need to remove the faction barriers that are currently left and just let us get on with it now. We are already in the same guilds the same PUG raids/m+ etc.
If that barrier was removed I don’t see any point in adding more factions but as long as they are just flavour fluff, I really have no objection.