Several states allow CRNAs to practice unsupervised. No states allow AAs to do so. Thus AAs have a leser scope of practice compared to CRNAs in the balance of things, especially since as Ortho pointed out, some states don't even allow AAs to practice at all.
Interestingly enough, the CRNAs advocate aggressively against AAs obtaining privileges, while the MDAs advocate in their favor. That is largely because AAs make no efforts to practice without anesthesiologist supervision.
Here's a link to an article in the ASA Newsletter concerning AA licensure efforts:
http://www.asahq.org/Newsletters/2004/08_04/stateBeat08_04.html