The $E^{\circ }$ value for the ${\text{Mn}}^{3+}/{\text{Mn}}^{2+}$ couple is more positive than ${\text{Cr}}^{3+}/{\text{Cr}}^{2+}$ or ${\text{Fe}}^{3+}/{\text{Fe}}^{2+}$ due to:

The standard reduction potential depends on the stability of the final state.${\text{Mn}}^{3+}$ has a $d^4$ configuration.${\text{Mn}}^{2+}$ has a $d^5$ configuration.Since $d^5$ is a half-filled configuration, it is exceptionally stable. The transition from $d^4$ to $d^5$ is highly favored, resulting in a high positive $E^{\circ }$ value ($+1.57\text{V}$).In contrast, ${\text{Fe}}^{3+}(d^5)\to {\text{Fe}}^{2+}(d^6)$ involves moving away from a stable half-filled state ($E^{\circ }=+0.77\text{V}$).