Why do very high oxidation states become harder to achieve after manganese?

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In the first transition series, the highest oxidation state for elements from titanium to manganese corresponds to the total number of 3d and 4s electrons. The stability of these high oxidation states decreases as one moves from titanium (+4) to manganese (+7).1

Following manganese, for elements such as iron, cobalt, and nickel, oxidation states corresponding to the loss of all 3d and 4s electrons do not occur. Generally, achieving very high oxidation states becomes progressively more difficult after manganese because the increasing nuclear charge causes the 3d electrons to be more tightly bound to the nucleus, making them harder to remove.1