Why does 2+ oxidation state become more stable relative to 3+ oxidation state for first row of transition metals? I viewed an image showing all the possible oxidation states of each element in the first row of transition metals, and the main oxidation states highlighted in a different colour. 

I noticed all elements Sc to Cr had in common a   3+ main oxidation state,
and from Mn to  Zn 2+ was the common main oxidation state.

The reason stated for this in my good textbook was 2+ oxidation state become more stable relative to 3+ oxidation state, I can't quite understand why.

So  Q : why does 2+ oxidation state become more stable relative to 3+ oxidation state  for first row of transition metals?

I had drawn the electron - in -boxes configuartion for all elements in the first row, and looked at Hund's rule and partially and fully filled orbtals, but I have not really seen a clear, fundamental explanation of why, for this question.
It is due to the difference in their second and third ionisation enthalpies. Due to the large difference in the second and third ionisation enthalpies arising from the disturbal of the stable half filled electronic configuration, +2 oxidation state is more favourable.