I suspect that Stirman was not considered for promotion to brigadier-general because, apart from the year he and his battalion spent in Mississippi, he mostly had an independent command, and therefore no opportunity to show his stuff leading a brigade. Of course, commanding a brigade was not a guaranteed ticket to promotion to general. Col. William F. Slemons (2nd Arkansas Cavalry), Col. William H. Brooks (34th Arkansas Infantry), Col. Robert G. Shaver (38th Arkansas Infantry), etc., all had exercised command of brigades as senior colonels, but none of them were promoted to general.

Also, Stirman was a proven commodity as a capable independent commander, and his superiors may have decided not to mess with a good thing.