Or, all he ever craved was the opportunity to steer Labour in the direction he wanted but never had the intention to lead them to power. Power is a young mans/womans game and as long as his replacement follows his direction he'll be more than happy in his retirement to the back benches/House of Lords/Comrades.
I suggest he has no sense of duty in to the country which would sustain through him through the slog that is government. To govern is to choose and to lead.
He's never really had to do either.