Here is my theory:
It MIGHT have something to do with Blizzard showing the Alliance as pitiful losers who do not really even WANT to fight and get genocided when they try to defend themselves, lying at the ground crying....while the Horde is standing at the beach shouting LOK TAR OGAR!!!!!!!!!
The depiction of the Alliance in general and Night Elves in particular has always been pitiful in this game. In the prepatch, it was at vomitting levels. Read both CE Novels...they are even worse. The Horde is all about dudes that like to get into fights and shit....while the Alliance one is about loss, misery, hopelessness, extinction and cowardice.
You cannot get any more biased than that.
Why should players want to fight for a faction that Blizzard has deserted a decade ago?
If Blizzard wants players to fight for the Alliance with pride, they should lead with example. They know how to do it. They do it for the Horde ALL THE TIME.
Again, just my theory.