The salts of Cu in +1 oxidation state are unstable because 

${\mathrm{Cu}}^{+} \text{has }3{\mathrm{d}}^{10}$ configuration

${\mathrm{Cu}}^{+}$ Disproportionates easily to Cu(0) and ${\mathrm{Cu}}^{2+}$

${\mathrm{Cu}}^{+}$ disproportionate easily to ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Cu}}^{3+}$

${\mathrm{Cu}}^{+}$ is easily reduced to ${\mathrm{Cu}}^{2+}$