I'm not sure if this is part of Slimbridge's reasoning, but Canadian Otters are much more active than Eurasians in my experience, while looking more like our native otter in terms of size and shape than ASCOs, for example (which the WWT keep at Martin Mere).
It may have been simpler than that - it could just have been that the Canadians were readily available and Slimbridge had an exhibit to fill!
