The stability of ${\mathrm{Cu}}^{2+}$ is more than ${\mathrm{Cu}}^{+}$ salts in aqueous solution due to -

The stability of ${\mathrm{Cu}}^{2+} \left(\mathrm{aq}\right)$ rather than ${\mathrm{Cu}}^{+} \left(\mathrm{aq}\right)$ is due to the much more negative ${∆}_{\mathrm{hyd}}{\mathrm{H}}^{\mathrm{o}}$ of ${\mathrm{Cu}}^{2+} \left(\mathrm{aq}\right)$ than ${\mathrm{Cu}}^{+}$, which more than compensates for the second ionisation enthalpy of $\mathrm{Cu}$. The ${\mathrm{Cu}}^{2+}$ ion has a greater charge density than ${\mathrm{Cu}}^{+}$ ion and thus forms much stronger bonds releasing more hydration energy.