My main is warrior and these are my thoughts.
I have noticed that a lot of tanks don't know how to tank. So this is maybe part of the problem with people not inviting them. I have a couple lower level chars and I am always baffled at how so many people playing the war class have no idea of how to play their role. So when people get to end game they have stopped running with tanks because they where useless at the start so why use them at the end.
I also think people get discourage playing war because the war is slower to level and harder to solo, so they just switch class.