Why is the heat of fusion of any substance generally lower than its heat of vaporization?

The heat of fusion is always less than the heat of vaporization because at the time of the phase change from solid to liquid, the molecules only require energy to escape from the crystalline network, but it preserves the other molecular junctions.

In the case of the phase change between liquid to steam, there must be a total breakdown of that intermolecular networks and therefore apply energy so that they are not again attracted by the conditions of their environment.