It boils down to the balance between ionization energy and increased electrostatic attraction of a more highly charged ion to solvent molecules.
If we look at ionization energies here and compare these with the maximum charge seen for forming a water-solvated metal ion, we see a pattern: the maximum charge corresponds to the point where the ionization energy surpasses 3000 kJ/mol, which is about 31 electron volts, at least for metals through the fourth period. In effect, the additional attraction to water molecules imparted by adding one more electronic positive charge to a metal ion is enough to balance 3000 kJ/mol of ionization energy. For instance, the second ionization energy of magnesium is 1451 kJ/mol whereas the third ionization energy is about 7733 kJ/mol, so magnesium will form solvated ions up to +2 charge.
Applying this to copper, we find that thecsecond ionization energy is about 1958 kJ/mol, so copper can form water-solvated copper(II) in aqueous solution. For zinc this second ionization energy is about 1734 kJ/mol, decreased from the copper value by using an electron from the 4p subshell versus 3d. But then the third ionization energy of zinc is about 3833 kJ/mol, so zinc is limited to forming aqueous +2 ions like copper. The solvating power of water molecules is enough to break into the 3d subshell of copper, but not the more tightly bound 3d shell of zinc.
Computations suggest that zinc actually can reach the +3 oxidation state in a very specific, nonaqueous environment using highly stabilized counterions with three negative charges. See the entry for zinc in Wikipedia's list of oxidation states.