In my opinion the main reasons for low take-up of the NPPL are:
1 - Commercial aspirations - obviously of no use there
2 - Misunderstanding of the NPPL - plenty of evidence of that
I have an NPPL, I've also had a PPL(A) - old-fashioned CAA type - my reason for taking up the NPPL was not medical it was the fact that it allowed me to do all I wanted to do with the least possible hassle. I've not changed my mind, yet.