Renal pathology generally deals with medical kidney biopsies and less with the more common neoplastic processes of the kidney which are more the realm of the general surgical pathologist.
The fellowships exist at many large university medical centers, but the fellowships are not in the same vein as the more organized and common fellowships (heme, derm, neuro, etc.) Many of the fellowships go unfilled every year and there is much more of a master apprentice relationship between the fellow and attending when dealing with the institutions biopsies and consults. Obviously, the big academic centers that are known as renal referral centers will have the largest variety of cases and are the most sought after fellowships.
My medical school's pathology department was chaired by a renal pathologist and he has a consult service for that region of the country and offered a renal fellowship which would fill maybe once every 5 years. My residency program has one attending who completed a renal fellowship and he deals with all the medical kidneys (we rotate with him on our renal pathology month).
While medical renal biopsies are fascinating and can be just as intricate and specialized as a heme or derm diagnosis (necessitating a fellowship) there are just not enough medical kidney biopsies to require renal pathologists in every practice/department. This pretty much relegates true renal pathology specialists to academic practice at major referral centers. That said, deeper knowledge of the pathology of any organ system is valuable to a group of pathologists (though probably not as valuable as GI, Gyn, or the like).
Also, renal pathology (in my experience) has less clinical utility than many other specialties. Most times the disease is already known to the nephrologist, and in the rare instances where you make a new diagnosis on an ESRD patient they already have a one way ticket to dialysis and no treatment regimen will derail that train.
