I can guess.  Typically, medical malpractice cases are very technical and require more time, money, and energy than a simple personal injury case, like a slip and fall case. 
 
For example, under MA law, it is not enough to show that you were injured as a result of a surgery performed by a hypothetical doctor.  Rather, the plaintiff must prove that, in performing the surgery, the doctor failed to exercise that degree of skill, care, and learning expected of a typical doctor under a the unique circumstances.  About the only way the plaintiff can satisfy that burden is to hire one or more experts, usually other doctors, to testify.  Those experts are very expensive, usually charging hundreds of dollars per hour.  For this and other reasons, lawyers will look long and hard at a medical malpractice case before taking it on.  Sorry.