1) Capacity. You don't see a ton of airlines flying that small of planes, and many of them are in sparsely populated areas, like Canada or parts of Australia, but not most of Europe.
2) SEA. The name of the plane says it all. Seaplane versions of planes you can buy normal are often more expensive, and the only routes they serve are ones with water but no asphalt. Most of Europe and the US have plenty of airports, but not a lot of empty water space, and most water is near a good airport. Canada and Alaska have inlets and coves, rivers, lakes, and not a lot of people in a very large area. It makes more sense in Europe to fly ATRs of Airbuses. It makes more sense in northern Canada to fly small planes that can land in odd places.
That being side, Canadian Airlines that serve mostly south Canada near the US border, such as Westjet and AC, fly larger planes.
3) Speed/range. Seaplanes fly for short distances slowly. There are so many LCCs in Europe that you could get the same ticket price on a longer route faster. Much faster. And in more comfort (though I'm not sure 28" pitch counts as comfort
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