As noted above:
1: demographics similar to the larger market
2: isolated, both for promotional cost reasons as well as determining effectiveness of promotion
but also:
3: relatively low real estate costs compared to larger cities like Toronto or Vancouver. A company testing out a new retail concept or chain might want to keep it going for several months or even years to determine viability. If it turns out to be a dud, the real estate/leasing costs are much less of a write-off.
4: this one I'm not sure about. I saw it mentioned in an article I read about test markets, but it was very old so it may not apply today (if it ever did). Anyway, supposedly people in Winnipeg are more conservative in their shopping/dining/spending habits, and tend not to jump on the latest thing. This is important because business owners need to have a solid grasp of the long-term prospects of a concept before expanding it nationally. It would be a disaster if the test seemed initially successful, but later revealed to have only been an initial rush as people flocked to try it out of curiosity, never to return. Krispy Kreme's entry into the GTA is a good example of this: at first it was an unprecedented success and they started a rapid major expansion, only to discover that business quickly dropped off as the novelty faded, and most of the stores were shuttered within a few months. So, the idea is that if it works in Winnipeg, it will work anywhere.