What specifically do you mean by saying that alliance is a "broken mess of a faction, community-wise"? What are you talking about? I've played Horde since closed beta but recently have played alliance alts with real life friends and I have not seen this difference you allude to. I think you're just saying shit that doesn't really have any meaning but I would be interested if you have some actual point.
Now for mythic raiding, because of the numbers, I get why horde is a better place to be. Outside that group, I just don't think people realize and they're used to playing horde so they just don't think about it. And without free faction changes from horde -> alliance I don't see enough people willing to pay or re-roll to take advantage of being alliance now.