Had he chosen to stand, being the intellectual force behind the Bevanites after Bevan's death, in 1960, he could well have solidified his position as the de facto leader of the Labour Left. That was when the future of the Left was made and, without Wilson standing, the three others who'd been rumoured to stand against Gaitskell (Anthony Greenwood, Barbara Castle and Richard Crossman) had their chance to become the de facto leader.
Thus, when 1963 comes around and Gaitskell pops his clogs, Crossman would have the best chance at defeating George Brown and claiming the leadership. From there, he has a good chance in 1964.