As number of valence electrons increases strength of metallic bond, enthalpy of atomisation increases.

Transition metals are referred to as the d-block elements of the periodic table; these are the elements that have incompletely filled d-orbital in their ground state or oxidation states. Transition metals, with the exception of (Zn, Hg and Cd) are quite hard and have a high melting and boiling point due to the involvement of a greater number of electrons from (n-1)d in addition to the ns electrons in the interatomic metallic bonding.

The high enthalpy of atomization is due to the unpaired electron in the d-orbital that favours strong interatomic interaction. Hence we can conclude that the greater the number of valence electrons greater the enthalpy of atomization and the stronger the resultant metallic bonding. Since the shielding effect of a d-electron is not that effective hence every new electron adds to the d-orbital increasing the nuclear charge and resulting in greater electrostatic attraction between the nuclear charge and outermost electron causing the formation of strong metallic bonds that require high enthalpy of atomization. 

Hence all the above factors prove that enthalpies of atomization of transition metal are high.