Why do transition elements have high melting & boiling point? 

Dear user,

The transition elements have unpaired d-electrons.  Due to these unpaired electrons they form strong metallic bonding. Consequently, they have high enthalpies of atomization. So, more amount of heat has to be supplied to weaken these metallic bonding. Hence, they have high melting and boiling points.

One thing which you must note here is that the strength of metallic bonding depends upon the number of unpaired electrons. Greater is the number of unpaired electrons, stronger is the metallic bonding. Hence, higher will be the melting and boiling points.

Hope, this helps.

Cheers!!