Dislocations move most readily in slip planes.
The reason dislocations move in slip planes is because their high packing density makes dislocation motion easy. The plane chosen here does not have good packing, and is therefore not a good plane for dislocations to move on. Furthermore, high packing planes have lower energy which dislocation motion favors. Dislocation movement is key to the ductility of metals. Ductility is the ability of a metal to plastically deform.