This is just a guess, but when there's no forced military personnel it's hard to say that we're forcing our young men and women to go die for an unjust cause. For a short while I remember hearing people discussing our military as "mercinaries" because they volunteered to die for money. But, they couldn't keep using that and saying they were "for the troops". I've actually heard some on the left of the aisle saying that they wanted to reinstitute the draft as a method of ending the war quicker, because more people would take interest in the "unjustness" of the cause if they (or their family members) were being forced into it instead of volunteering into it.