Chairpersons? I'm fond of just "Chair" myself, but YMMV and all that.
Although like all oversimplifications, this one is too, but generally academic chairs at the larger pedi programs are chosen in significant part for their research background and history of federal funding, especially NIH. Although this can exist for Academic General pediatricians, it is somewhat less common.
For those less academic programs, there is still the reality that most folks in the academic hierarchy are specialists, not general pediatricians. There are exceptions of course, but even then, many of those folks (but not all), may not be seeking out academic leadership jobs.
Now then, the real question is why would any sane specialist wish to be a Pediatric Department Chair? Let me know when you solve that!