Electronic configuration of \(\mathrm{Mn}^{2+}\) is \(3 d^5\). This is a halffilled configuration and hence stable. Therefore, third ionization enthalpy is very high, i.e., third electron cannot be lost easily.
Electronic configuration of \(\mathrm{Fe}^{2+}\) is \(3 d^6\). It can lose one electron easily to achieve a stable configuration \(3 d^5\).