Why does copper have a more common oxidation state of +2, but only one valence electron?
It's an energy balance. Donating additional valence electrons to make higher oxidation states costs energy. This has to be made up with the electron affinity of the nonnetal and greater electrostatic attraction to negative ions and dipoles. In the case of copper it usually works for donating one
There are exceptions. With highly electronegative oxygen and fluorine atoms there may be enough electron affinity and electrostatic attraction to draw off additional
On the other hand, low-electronegativity "soft base" nonmetals often have enough electron withdrawing power to bind only the energetically "easy"