How is supination made difficult with a radius fracture near the wrist?

Supination rotation. of the forearm to place the palm up often is diminished even with simple fractures that are casted because when one sits in a cast they tend to hold the forearm pronated when elevating, and for fractures that typically displace, slight pronation and flexion is the preferred casted position. Therapy is the first line of rx., encouraging motion , and doing exercises often helps to start.
It depends. There can be several reasons for this. 1) the ligaments attached to the broken pieces are stressed when supinating - if the fracture is not healed all the way, this will hurt. 2) a piece of fractured bone may not be lined up or healed correctly, which may cause the joint between radius and ulna to be locked up or badly aligned. 3) it could also just be normal stiffness after a fracture.
Wrist. Supination causes stress on the radius and thereby causes pain. This limits supination and makes it more difficult.