Because as much as we are out for blood and rvr, we do and will have members who don't want to spend every second fighting for their lives. Some people just don't find that relaxing. I respect that, and we wont require anyone to roll on a server they don't want to.
Also there is the added advantage of being able to pick our battles. A core server allows us to do that.
I think there are some mistaken impressions among the general community that a core rule set server in WAR is going to be like a normal WOW server and thats just not the case. over 90% of all the terriitory at max level in WAR is going to be RVR territory even on a Core Rule Set Server, so there will be no lack of war going on in the end game.