Question

why is the latent heat of fusion lesser than latent of vapourisation?

Answer 1

Latent heat of fusion is needed to break down the intermolecular force of attraction between the particles of solid which very tightly packed. But in liquid the particles are not so tightly packed as solid. So less latent heat is required to break down the cohesive force in liquid.

That is why latent heat of fusion is lesser than latent heat of vaporisation.