Why Zn has highest ionisation enthalpy in 3d series?
There is band structure formed in the metallic lattice  in the crystals of d and f block elements due to partially filled orbitals. But as you move along the periodic table to 'Zn' , you will find that the orbitals are completely filled hence there is no possibility for inter-orbital mixing. Another thing worth noting is that due to the filled d-orbital, zn and copper behaves more like s-block rather than d-block elements.

"The filled d-shells in these elements contribute little to interatomic interactions, which are dominated by the s-electrons through metallic bonds. Unlike metals with incomplete d-shells, metallic bonds in copper are lacking a covalent character and are relatively weak. "

An alternate explanation can be on the metallic lattice:
Copper has a face centered cubic structure and zinc has a hexagonal close packed structure

reference : wikipedia