What qualities does the manager typically posses that the employees reporting to him do not have, that makes him a better candidate for project planning
Typically, leadership. But most often, there are a number of folks in the organization who could do a credible job...why would he hire anyone less capable than that if he can get them?
Better candidates are in the eye of the beholder. One has to be careful with that word better. Generally, in choosing a project manager, there are a number of persons who are equally fit, and the choice is often a matter of throwing dice. This is not a precise deterministic science...one has to forecast future problems, which is close to witchcraft in itself, and then suppose which candidate can deal with those supposed problems.