The soles of the feet have very thick skin. The nerves in the biopsy have to be easy to access. Also infections are more common in the feet and since some PNers are diabetic, injury to the feet is avoided when possible.

Many pains felt in the lower legs may arise actually in the dorsal roots along the spine. These are the beginning of the peripheral nervous system. When these nerves are damaged then the nerves farther down, start to atrophy. This is because the feedback from the nerves in the dorsal roots are keeping those in the farther periphery working.

This thread explains some of these details:
http://neurotalk.psychcentral.com/thread168511.html

This thread explains some new testing (MRI) for dorsal root damage, and further goes on to explain the biopsies:
http://neurotalk.psychcentral.com/thread147771.html
It is a long technical thread, so read it slowly and perhaps in bits to get the gist of what it all means. The dorsal roots are typically repaired in most people after a damage (this is where shingles causes pain for example), but some lack the DNA genes to do so. Therefore stem cell therapies are being looked at to fix this damage now.

So if the dorsal roots are not working properly you may feel pain anywhere along the paths of those peripheral nerves.