Part of the answer is easy. Teachers are professionals, and they have been unionized for almost a century. Members of the nursing profession have been joining unions for at least 70 years and have been in the AFT for two decades. Professionals are just as interested in decent working conditions and decent wages as other workers. They don't work under a contract or engage in collective bargaining.
Psychologists are a perfect example.
Like nurses and doctors, they find that managed health care has put them into a very tough position. Their patients, many of whom are suffering from debilitating illnesses, depend on their therapist's professional expertise to help them recover. But managed-care companies have come between the professional and the patient. On their own, psychologists are hampered in their ability to act in the best interests of their patients. Union affiliation will help them get back their professional autonomy so they are once more free to act according to the standards of their profession.