The real reason is quite obvious. It's because the alliance men aren't satisfying the alliance women
Male humans are always intent on running off to war and serving the king leaving their wives home alone.
Male night elves are either just sleeping all day or fixated now on sylvanas instead of their wives
Male draenei are to busy worshipping the Narru to care about their wives (and don't get me started on the light forged ones)
Male gnomes are just to short in every degree to do anything, plus to focused on machines rather then ther women.
Male dwarves come close to being good for it, as long as you catch them before they pass out from drinking.
Male worgen, the alliance women pretty much decided they don't want to get cursed or have werewolf babies.
Male pandaren are more interested in food, old stories and sleeping then doing anything else. (Same applies to horde ones)
Male kul Tiran have sadly developed scurvy making them less desirable in the bedroom.
Male void elves are heavily into tentacle hentei and this is a bit to far for some alliance women.
Male dark iron similar to regular dwarves but with the added fact they keep burning down the houses of alliance women with their flaming beards.
Male mechagnomes replaced it with wrench so serves other purposes now.
This is why the alliance women figured screw this, and are having extramarital affairs with the horde boys.
Male orcs have no problem unleashing the beast all night long, same with mag'har, although regular orcs can go the extra with blood fury.
Male tauren can give alliance women a touch of nature and the great outdoors, same with highmountain.
Male trolls are just crazy under the sheets, plus they have no problem mesmerising alliance women with dah voodoo, same with zandalari.
Male forsaken.. okay I got nothing they probably don't even have anything left down their, rotted away.
Male blood elves are.. okay they are pretty vain and would take do long preening themselves before getting to the act.
Male goblins are pimps, if you can afford to pay them.
Male nightbourne would probably want some freaky cosplay before they get to the act, but they get there.
Male vulpera, the problem they face is the alliance women think their so cute they just want to pet them rather then get to anything right away.
So when it comes down to it, the women of the alliance are more likely to burn the midnight oil while their husbands are away with several horde boys sneeking in after dark.
This is the real reason alliance based factions opened their doors to the horde