The shortage is due to several things, IMO.
One is the reduced salary for peds neuro compared to adult neuro, though that's a problem for all peds specialties.
Another is what I think is a much higher psychological burden of peds neuro - you have to deal with abusive/neglectful parents (or OTOH helicopter parents who think every hiccup is a seizure and merits hospitalization and million dollar workup), grim prognoses in small children with trauma, anoxic, or congenital/metabolic diseases, and many layers of social work that don't really exist in the adult world. The closest I came to not choosing neurology as a specialty was after spending 2 weeks rotating in peds neuro as a 3rd year med student.
A third is the convoluted training pathway - I think this is improving with more programs going categorical, but historically some applicants have had to essentially apply to pediatrics, lie to their program directors for 2 years about their interest in peds neuro, then suddenly drop out of peds and join the peds neuro training pathway. I even know people that started out as peds neuro, but then were only a year away from finishing gen peds training and decided to not do an extra 2 years of training to make the same income in the end.