As grim as it is, Alliance Players only have the Alliance to blame. No amount of external incentives will make the Faction itself more popular - or to be exact, more popular with less casual players.
The Alliance itself would need a lot of changes to make it more attractive to more players.
That, and maybe playing on a realm that isn’t one of the most Horde popular in the region? Aren’t the RP/RPPvP realms not more scewed to Alliance? Plus, with the way they work in War Mode (with their own phases) they usually have a vast Alliance majority in the open world.