Anyone that says the imbalance is a racial issue is only partly true. Yes it was true many xpac ago where horde racials were so good many pvp players transfered then guilds transfered and since alliance number dwindled and as guild found it harder and harder to recruit, guilds tranfered to horde more and more.
My guild used to be alliance and 10 years ago, give or take a few years, we transfered horde to break the disastreous rtecruitment boss on the alliance. with cross faction raiding many of my players will move some characters to the alliance. If blizzard makes cross faction guilds then about 70% of my guild will main alliance toons.
We are talking 2 to 7 characters per raider for a total of 76 transfers total (we did a guild survey).
I am fairly certain that should blizzard finally creates cross faction guilds, the number of alliance guilds will steadily increase again.