I think "idiopathic" is misleading.

Neuropathy that arises from other diseases is basically a secondary syndrome. In medicine there are primary and secondary illnesses.

Because there are over 100 types of neuropathy, the word idiopathic was reserved for those that don't have an obvious cause. Doctors vary in their approach to finding the causes.
Some don't want to bother, and others will search until they find a cause.

A primary disease example is this:
Gout can be primary...meaning it exists by itself (this is a metabolic error of metabolizing purines).
But Gout can be secondary, meaning it may arise following chemotherapy for leukemia. When millions of white cells flood the system, purines are released, and gout can occur. This will go away once the purines return to normal.

PN is like this.
Primary PN would be like CMT.
Secondary PN arises from diabetes, thyroid disease etc.
(not all diabetics get PN, by the way).

Idiopathic is just a fancy word. When doctors fill out medical insurance forms there are codes for the diagnoses...so names have to be given for them. If no cause is known at the time, something has to be written down. Doctors have to have a word for everything, it seems.

It would probably be more clear, once a firm diagnosis is made.