Many airplanes (not sure about the CRJ) also have a center wing tank, which is directly under the fuselage, so would be kinda hard to separate cleanly. Also, explosive bolts might start a fire that wouldn't have started without the bolts...
In larger aircraft that do have the ability to dump fuel, the reason is to make the airplane lighter so it does not have do an overweight landing and the subsequent high-cost overweight landing inspection. It is not done to reduce the probability of a post-crash fire.