I think Sonia Gandhi picked him as the PM because he was a 'good man', just too good and too decent to ever assert that he is the PM, and nobody should interfere with his office. He remained silent, obedient and allowed himself to be dictated by the 'high command' aka Sonia Gandhi, allowed the PM's chair to lose its gravity, and allowed Sonia Gandhi to enjoy power without responsibility and run her personal agenda of making her son the PM of India.
I strongly think that leadership is a quality that either you have or you don't...sure one can train himself to become a better leader, augment his leadership qualities, but one need to have those qualities, confidence, assertiveness, and natural inclination to take charge, and make hard decisions, to begin with. That's why many technocrats with excellent track records in their domain fail miserably in leadership roles, because that requires different human qualities and a different personality altogether. No doubt PM's post requires extraordinary level of leadership skills, and people who have risen to a position on their own capacity in politics, i.e. proved their leadership qualities, are better suited for PM's role (and CMs' role too). A top down approach is likely to fail here.