Having read Storm of Steel and making my way through Infantry Attacks, its evident that both Rommel and Junger were competent and successful junior officers, both receiving the Blue Max by the end of the war.
Given their talents and accumulated experience, and given the high turnover in infantry officers in any WWI army, why did they still finish the war as company commanders?
Was promotion especially slow in the German army at the time (and if so how and why) or where there other reasons?
It would be important to keep in mind how utterly large armies were in the First World War. When dealing with formations on multiple fronts numbering in the millions, even talented officers might not accelerate through the ranks, despite high attrition. Another factor to consider is the caste system of the Imperial German Army. Usually, preference was given for promotion to officers of particular family backgrounds, and more senior appointments were usually reserved for those who had attended the Staff College. Another thing to consider is that both Junger and Rommel were very good Company commanders, and may have well been left in that position because it remains necessary to have competent officers at all levels, and moving everybody up a notch when a vacancy becomes available can actually be detrimental to efficacy. Notwithstanding, the need for officers in the Imperial German Army did become acute, especially late in the war, so much so that they developed a class on non-commissioned officer to the level of platoon command for the lack of subalterns (the most junior officers). (see Holmes, Richard "Tommy pp. 536-37)