There is perhaps a suspicion among leftist elites that the volunteer military is composed of the more TRADITIONALLY patriotic and that a diluting of the ranks with, ah, REGULAR Americans will leave the force more likely to follow orders that the more tratitionally patriotic may balk at.
First thing that popped into my mind, actually...
Another view is that the all volunteer military, is destroying it, and ending the history of the traditional American fighting machine.
When you need 42 year old grandmothers and single moms to fill your ranks, then they will eventually shape it into their own image, and shape it to serve their needs and demands and wants, something that 200 years of man draftees couldn't do.