The stability of Cu²⁺is more than Cu⁺ salts in aqueous solution due to

The stability of $C{u}_{\left(aq\right)}^{2+}$ rather than $C{u}_{\left(aq\right)}^{+}$ is due to the much more negative Δ_hydH^o of $C{u}_{\left(aq\right)}^{2+}$ than $C{u}_{\left(aq\right)}^{+}$, which compensates the second ionisation enthalpy of Cu.