Experience and knowledge about lawyers and firms. Lack of teaching and course development skills. They may want to cross sell the firm unneeded consulting services. The rainmaker's style may not be appropriate for everyone and could cause resentment and negative vibes. Often, they themselves do not know nor can they teach the secrets of their success.
Teaching rainmaking by mentoring is largely a myth.
They have written books and teach marketing in business schools. They understand the concepts. These people are usually good at concepts and theory but frequently they lack practical experience. It is often said that they are teaching because they can't make a living themselves doing what they teach others to do. They may have been clients and may understand the buyers' motivations and needs.