Perhaps like the Jacobs shown on the left (photo courtesy of the Alberta Sheep Producers Assoc.) they aren't very practical in areas that require indoor feeding at least 1/2 of the year (such as Vancouver Island with a climate poorly suited to year round outdoor production of any livestock). These animals are poorly suited to being constantly soaked to the skin and what land animals are? Feeding these sheep out of feeders nearly always proves impossible with lots of death losses and serious injuries reported.
Calling an animal "heritage" or "rare" seems to have become a good way to attract buyers who are looking for something unique as more of a pet than actually raising sheep for meat or wool. I have no problem with this except the implication that I attach to these terms suggests that these breeds were common in Canada but for some reason have dwindled. When it turns out they didn't ever exist in North America until the 10 or 20 years makes me question what this is really about.
There are literally hundreds of breeds of sheep (maybe even thousands) all of which have come from the original sheep ancestor known as the wild mouflon back in 11,000 to 9000 BC. Sheep were one of the first animals domesticated by humans, and there is evidence of sheep farming in Iran dating to that time. Now if any of those sheep still existed they would really be "heritage" but not to Canada!!
Sheep like all domestic animals have been maintained since the beginning of agriculture because they are useful. Those most useful were selected and bred eventually resulting in various breeds best suited to particular climates, and for particular needs. As times changed so have those needs as well as production methods, land prices and other costs of production. Those best suited are the ones that are continuing to be selected for and even among those breeds at the top of the "popularity list" such as Dorset and Suffolk sheep, the sheep themselves have changed to suit the changing agricultural practices and pressures and more and more importantly the demands of the consumer and will continue to do so. The Dorsets that were first imported into North America no longer exist anywhere but if they did they would be poorly suited today's consumer needs and preferences.