--because the original normed figures that intraepidermal nerve fiber density ranges were based on (the original work was done at Johns Hopkins) came from groups of subjects in which the density was measured at lower leg, upper thigh, and right above the elbow.

The original Johns Hopkins researchers chose these places as they are standard places of hairy skin with at least a thin fat layer, meaning it was easy to take the samples and there were few structures to get in the way or compromise the measurements.

It is possible to take samples from anyplace there is hairy skin--I know a few people who've even had it done on the face--but it's much harder to interpret the results in these areas as normative levels of nerve fiber density are not well known.