I think that idea that the boats were too heavy is correct.
Rafiki weighs 18 tons. Its like a super tanker on the canal systems. 1. the wash is substantial especially in narrow water ways and 2 the screw / engine isn't powerful enough to stop the boat at all.
As an owner its quite rewarding as you really have to think ahead, plan everything your doing. Entering a lock, it may appear the boat is going way too slow and cautiously but its the only way to not over cook it. And when it goes wrong it can cause quite a bit of damage, not to the boat, but to other boats and property (and people, my wife badly broke her arm trying to push off against another boat).
On the plus side, it's extremely stable. Staying board during rough weather or when other boats go past, she's rock steady. You may see light weight GRP boats bobbing about when Rafiki barely seems to be moving at all. And its surprising insulating. Staying aboard in the winter isn't a problem at all. Even though it doesn't have a fancy diesel heater. As for repairs, I haven't had to repair anything.... yet. The comment about screwing into it or modifying the hull in any way is valid though.
So the weight issue is probably the reason it didn't take off, especially for the rental market.