That's an interesting question. Or in the same logic, we can still ask, why most JDs but not PhDs teach in Law School?
JD is equivalent to a Master? OMG, a JD degree requires more than 90 credits to graduate. How about a MA or MS degree?
It seems that people need to dig more about the history of higher education.
Doctor, a latin word, which has origin in Arabic, means the qualification to teach law and offer legal advice. In the earliest university in Rome, only three majors (could be four) can be granted a Doctor degree: Theology, Law (civil and canon), and Medicine. Doctor refers to a professional qualification to teach and practice. At then time, the highest degrees all other majors can be confered on is Magistrate or Master. In many countries, law prohibits any graduates other than those in the three majors from using the designation of Doctor. Why? because other disciplines (all under the general name of philosophy)were highly soft and socially useless. However, those students in philosophy (either in natural or in moral) were so frustrated that they added a strage suffix Master and Doctor to their names. Till 19th century, it is Leipzig University that granted the first Doctor of Philosophy or PhD in the world. Obviously, it tries to euqalize the majors in philosophy to three privileged subjects in terms of social recognition. Also, enhancing master to dotor also belies that the scholars in philosophy intended to imitate those three highly developed subjects to Professionalize their own discipline. This is why they use the defining word Philosophy, which indicates their intellectual feature as well as historical imprint. But don't get it wrong, PhD in any sense is not higher than Doctor in other professional majors.