My guess is that the people who fly helicopters tend to be wealthier and more willing/able to pay. I think snobbery is involved - if you can afford it and don't complain about it, it indicates that you're a rich person and gives you access to an elite social circle. It seems to me that student helicopter pilots generally fall into two categories (1) those who aspire to a CPL and (2) those who aim to buy their own machine and have the money to do so. There appear to be fewer recreational fliers in the rotary world who would be happy to hire the club machine for an hour once a month.
Other reasons for the difference in Instructor pay? Rotary Instructors fly fewer hours than their fixed wing colleagues?