Why are lawyers not called "doctor"?
The American Bar Association has a nice explanatory essay that answers exactly this question.

In essence, in the United States, the J.D. degree is quite new, with lawyers having previously received a professional degree that was not called a doctorate.  As such, it has generally been considered problematically confusing to call a lawyer "doctor" and also possibly unethical as undue self-promotion (lawyers used to be prohibited even from advertising in many areas).