Not to mention the worst part about being on these wildly unbalanced realms is the fact that the lack of PvE progression doesn't help either. Obviously people would be attracted to the faction that has a guild that's 7/7 Heroic Firelands compared to one that's only 7/7 Normal.
I assume that the AH and the economy in general on those factions is unexistant as well.
And sadly this trend isn't just limited to Illidan and Mal'Ganis, check out a few of these other realms:
Thaurissan-US: 2 Alliance guilds downed FL content compared to Horde's 79.
Cho'gall-US: Realm got attention with The Resistance movement on Alliance side, but kinda died out.
Chromaggus-US: Best Alliance guild is 1/7 normal, and that was in a partial PuG.
And there was that one Russian realm that has 146 Horde guilds downed FL, and 0 Alliance ones.