Account for the following :
E° value for the Mn³⁺/Mn²⁺ couple is highly positive (+1.57 V) as compared to Cr³⁺/Cr²⁺.

Mn²⁺ exists in 3d⁵ configuration which is a half-filled configuration which provides extra stability to the Metal Ion. While Mn³⁺ will exist in 3d⁴ configuration which is less stable than 3d⁵. Hence the conversion from 3+ to 2+ is very feasible. Hence E° value is more. while in
 Cr⁺³ exist in 3d³ half-filled d orbital (3 e; electrons in t₂g ) extra stability is attained by Cr⁺³ than Cr⁺². Hence Cr⁺³ →Cr⁺² is less feasible. And hence it has less reduction potential as compare to Mn⁺³/Mn⁺².