As I understand it there were 2 questions. First was on changing to PR, and second was what form of PR. The 42% response rate was to the first question, much lower response to the second; probably those who wanted FPP didn't respond to the second.
There were three PR options but two of them still had open issues that were to be determined later.
Makes sense that many FPP voters didn't respond to the second question, why would they legitimize a system they didn't want in the first place.