I guess it's because of the same reasoning only a few guilds/people do WoE. BG requires team effort, and as such, it requires a leader of some sorts. If there is no such person, or if there are two or three who want to be that leader, things tend to get chaotic, and people lose fun.
Actually, if it's because of rewards, a lot more people should to BG instead of WoE, since you can BG ALL THE TIME, while there are only two WoE days each week to get a castle (and the treasure chests).
So I really doubt it's because of the rewards, and I don't want to make the rewards ridiculously high. It's not my problem if there is content (no matter if custom or official), but people don't use it. Even if it's just about Gravity rewards.
We could also lower the number of players needed for each match, but I'm also against that. The whole "feeling" of BG gets lost when there are just a mere 5 people fighting against another team of 5 people on a big map.
Maybe if you want these things to change, how about inviting really new players to the top tier guilds, instead of only keeping vets in there? That'll make the newer players get used to things such as WoE or BG or PvP.