Serious off-road cycling is very demanding of skill and fortitude on the part of the riders. The greater length of a tandem seriously reduces its maneuverability. The bumpier surfaces encountered off-road are much more punishing to the stoker's rear end than the mere potholes encountered on pavement. For these reasons, I would not look for off-road tandeming to ever become a major niche in cycling. I don't think so.
Some people buy tandems because they are so fast.
Pat and Chris Typical tandem purchasers are married couples. Pat is a serious cyclist, Chris isn't. Perhaps Pat feels guilty about going cycling without Chris, perhaps Chris is jealous of Pat's time spent cycling. They have tried cycling together on solos, but Pat hates having to constantly slow down to wait for Chris and Chris feels pressured to go too far and too fast. The tandem is the obvious solution to the problem, so they buy one.