I am fairly sure that your otters are quite literally the only mammal species you have had on public display since I started visiting Hamerton which I have never spotted, with the exception of one species which went on-show and off-show between visits

which says a fair bit about how shy and reclusive they are!
Interestingly, the species
does seem to breed pretty regularly in private hands - this being the main source of the influx of Palawan Binturong into UK and European collections - so the reason for the low rate of breeding quite possibly is, as you note, down to a lack of any real interest in doing so rather than a lack in husbandry knowledge.