I don't think the Pain doctors themselves know exactly why SGB's help CRPS pain. However, it seems to be they are being dictated to by insurance companies, and unless you are Rich as Croceus to be paying for everything yourself, one's medical treatment is somewhat affected by what your medical insurance provider guidelines.

Most Med Ins companies want to see 6 SGB's before they will entertain the thought of a Spinal Cord Stimulator or some other really expensive treatment like a pain pump. It has some positive predictive value that the device would work.