Because:
M(H20)6 2+ + 3en ---> M(en)3 2+ + 6H2O
i.e. 4 to 7 species, therefore increase in entropy is highly favourable.
Regarding the other point, I think it's just basic electrostatics. If you're pairing M(III) with say an edta 4- hexadentate ligand, then pure electrostatic attractions will be stronger when the metal is in the higher oxidation state.