C.E. Evans wrote:
It's probably because quite a few people would still consider it an, um, acquired taste...
I couldn't agree more.
Also, as mentioned above, there are many people nowadays with a peanut allergy. It'd be extremely difficult to make sure that there are not only peanuty burgers but also ones without the slightest trace of peanuts in and on them. You'd need two kitchens, two counters, different plates and cutlery, different ovens, frying pans, dishwashers, and 2 sets of staff.
It'd be too expensive, compared to the tiny profit they could make.