I think there are 3 reasons for this:
1. Not a lot of people go into neuro expecting that pain mgmgt will be a big part of their practice. I know I didn't. This is a huge "flaw in the system" stemming from the structure of most neuro training programs, which in med school and residency focus on hospital-based neurology, i.e., stroke and other acute events. If more time was spent in an outpatient setting during training, I think people would recognize that at least 50% of general neuro is some type of pain issue (headache, back pain, peripheral neuropathy, etc).
2. Many (most?) docs simply don't like deaing with chronic pain patients.
3. Interventional pain mgmt (which is where the real $$$ is in pain) has long been the realm of anesthesia and/or rads. They're not going to be happy to give it up, and again, you just don't get exposed to it in med school/neuro residency.