@jalcorn, You ask a good question but I'm not sure anyone can answer it. A lot of us that have been given an idiopathic diagnosis would love to know for sure. Here are a couple of research articles on the topic.

"What is the most common cause of axonal polyneuropathy?
Diabetes is the leading cause of polyneuropathy in the U.S. About 60 to 70 percent of people with diabetes have mild to severe forms of nerve problems that can cause numb, tingling, or burning feet, one-sided bands or pain, and numbness and weakness on the trunk or pelvis."
--- Peripheral Neuropathy: https://www.ninds.nih.gov/health-information/disorders/peripheral-neuropathy

"Diabetes, HIV infection and alcoholism can cause several patterns of neuropathy. They most commonly cause a distal, symmetric axonal sensorimotor neuropathy. The second most common presentation in these conditions is a small-fiber, painful neuropathy."
--- An Algorithm for the Evaluation of Peripheral Neuropathy - AAFP:
https://www.aafp.org/pubs/afp/issues/1998/0215/p755.html