a) They are heavily regulated and moving outside the regulatory framework is subject to vigorous prosecution. AirBnB is in a comparitively lightly regulated industry and its transactions can remain private.
b) These industries are already profitable so the most likely perceived result of disruption is a reduction of profits.
c) The amount of cash already attracts many players to the market who can focus on sales - lawyers and doctors like a professional sales force and personal attention - rather than innovation. Little egotistical professional service companies are as problematic as big dumb one's. Maybe worse because they are about more than the bottom line.
d) These industries have better ways to make money than software services. Insurance companies don't reimburse for medical records on an iPad. Lawyers bill by the hour.