Direct currents are very difficult to stop or interrupt when compared to alternating currents. Alternating current sources reverse the flow of current many times a second (in some locations 100 times a second on 50 Hz systems). Each time the current reverses, it goes to zero in magnitude. A zero current is very easy for a melting fuse to stop or interrupt—it is already stopped, and there is no force trying to sustain an arc across the fuse element.
DC currents, as the name implies, are currents that travel in one direction only. They do not reverse. Fuses bear the entire burden (with no help from the current) of acting to stop these currents. The internal elements of a fuse must react to an over-current condition (usually by melting) and as they react, they must do so with enough capability to interrupt the current from flowing while extinguishing any arc that might form. DC fuses are relatively sophisticated devices that have many different internal elements that must work together. The complexity of DC fuses sometimes results in a higher cost than AC fuses that may contain only a single element. There are fuses with equal AC and DC voltage ratings, but the DC interrupt rating is significantly less than the AC interrupt rating.