The first is the narrowness of the promotion pyramid. At every level, for every 4-6 officers of your rank, there is only one spot for the next higher rank. At the low levels, this is not a terrible problem, as many officers do their commitment and leave the army, but it makes it nearly impossible to achieve Colonel.

The second is the up-or-out policy. As an officer, if you are passed over for promotion twice you are kicked out of the Army. This, plus that first policy combine to weed out the risk tolerant and unlucky. With so many competitors for each slot, any blot on one’s record is grounds for getting passed over… and in a peacetime army, there is really no way to distinguish one’s self from one’s peers in a meaningful way.

The last factor is the political nature of upper level – Colonel and above. Essentially, the primary skill of a general is public relations and politics, not just with other members of the Army, but with members of Congress and the White House as well.