Vanilla WoW had a limit of 3 BGs you could queue at the same time. It made sense because there were only 3 battlegrounds in the entire game.
Turtle WoW added 2 BGs so there were 5 BGs you could queue. However, the limit of BGs you could queue wasn't increased to 5 and stayed at 3. This led to situation where players who wanted to PvP all the time without much waiting, could sign up to only 3 battlegrounds. Because of that, players usually chose the most played BGs like WSG, AB and AV.
Unfortunately this led to situation where other custom BGs were barely played (Sunnyglade Valley, Arena, Thorn Gorge). This wasn't fully natural decline, it was partially caused by this 3 slots limit.
Suggestion:
Increase the limit of BGs you could queue to 5.
I believe it would possibly revive dead BGs. I know that some PvPers could queue all 5 it was possible.