The displaced fracture: Casts are best at applying bending loads and not good at distraction of shortened fractures. Because of their significant transverse component (which prevents shortening once reduced), bones which have failed in bending are the best suited to closed reduction and casting. Oblique, spiral and comminuted compression fractures won’t hold themselves out to length anyway and so frequently aren’t worth trying a closed reduction/casting on. In bending failure of the bone, the periosteum and soft tissue is ruptured on the “tension side,” and remains intact on the “compression side.” This intact soft tissue on the compression side now holds the bone in an angulated position as below (it’s on the concave side).
Applying traction: If you only apply traction, the situation below occurs where the intact soft tissue on the concave side, being stretched across the hypotenuse of the triangle, is too short for you to obtain sufficient length to reduce the fracture.