What also promotes DK tunneling is the fact that DKs can root/grip an entire team. There is no point to chasing down a healer when the DK can pro-peel the whole game long. Face it--yes, DKs are susceptible to some CC and are nearly immune to others. That is what is called "balance." I really don't see why any class would need to be immune to all CC while also having the most incredible raw damage around.
Okay, I am a bit biased. As far as I am concerned, the class is still paying penance for s5.