I don't know much about the physical sizes and configurations of different types of brake. The standard freight-car truck frame is designed for use with clasp brakes (with places to mount brake cylinders and space for the brake rigging): would it have to be modified to accommodate the sorts of disk brakes often used on passenger rolling stock?
The North American freight railroad system depends on interchange: cars belonging to one railroad (or other company) operate on the track of other railroads, and can be repaired by other railroads' mechanical staff. Introducing disk brakes on freight cars would probably involve re-writing the agreements and conventions governing interchange: think in terms of years of lobbying and committee work. If there are disk brakes somewhere in use on otherwise conventional freight trucks, it might be on isolated systems that don't interchange with other rail carriers (something like, e.g., the Western Australian iron ore, mine to seaport, railways).