Melting point of crystalline solids
Remember that temperature is a macroscopic quantity, telling the heat content of system. So if the crystalline solid is at its fusion temperature, and every atom in the crystals have equal interaction energy with each other (ignoring all defects of lattice), the whole solid should fuse simultaneously, thus having sharp melting point.
On the other hand, amorphous solids don't have uniform lattice and interaction energies of atoms/molecules may differ. So even at the same temperature, the whole solid might not melt, so we say amorphous compounds melt at range of temperatures.