One theory for why SOEs are less efficient than private firms on average is that industries are often nationalized as a way of saving them - imagine if the US auto industry were nationalized right now, and you have the idea.
I don't think SOEs as currently practiced do represent what most on the left think ideal - the idea of worker management is clearly in there, and there is some evidence that worker management leads to higher productivity - but the inefficiency of these firms is problematic for us if we advocate nonmarket means of coordination and can't provide good theoretical reasons for their overall (if hardly universal) inefficiency.