Woe isn't fun for most people mainly because of the already mentioned monopolization, making it more of a battle between major alliances rather than a war between the whole server for the emp.
Opening up all 20 castles like the official servers will make the gameplay more hectic and allow for more strategic plays to be applied, also allowing for smaller alliances to participate in woe since holding 20 castles is a lot harder than holding 2 for the big guys no matter how you look at it. Right now, there's no fun in fighting a battle you know you won't win, so people don't bother with woe.