[WS:] An interesting question. My guess is that it has something to do with the feminization of this occupation from early on, as opposed to, say, doctors or engineers. The same can be said about social work, I think. Another possibility is the public ownership of educational system (below the university level) vs, private practice of medicine or engineering, at least in the US.
Another observation - professionalization proceeds differently in different countries, depending for a large part on the government control of the professional practice. For example, in Eastern Europe medicine is government regulated so self-regulation - the raison d'etre of the professional project - is almost non-existent. So the status of an occupation depends not only on the claim to expert knowledge, as you correctly point out, but also how it is regulated.
Wojtek