Kristi's on the right track, I think. In most states, getting your salesperson's license is easy; all you need is a few hundred extra bucks, the ability to pass a multiple-guess test, and a broker willing to sign that he or she is your "sponsor" (so to speak). In my state, it means that any idiot of legal age who has never been convicted of felony fraud can become a real estate agent. Compare that to the standards required for our accountants, lawyers, physicians, or even appraisers.
Most folks who go into real estate have no sales background. They don't know how to foster or maintain the sorts of professional relationships that lead to sales.
It's surprising, actually, that the failure rate isn't even higher.