I would say its a combination of lack of suitablly equipped rental aircraft (and maintained to such a standard that you would put your trust in the kit) and the, as Bose says, there is a reluctance for schools to sign you out.
I would hazard a guess that most people renewing IMC/IR quals are either going for the airlines or are owners and therefore more likely to use it. Maybe any FIs on here can give some info on that one for their own experience.
J.