The problem comes down to gear and how fast people can get it. If illum was faster to get gear by killing and attacking or defending than bgs people would be out there attacking everything thats red. However, this isnt the case. BGs are the best way to get gear with the supplement from Illum.
Long story short, it isnt benefiacal to kill the other team in open pvp as it hampers your actual progression. Design Flaw.