Shane Dingman's recent Globe and Mail article about "the investment case for mid-rise condos" is a good summary of why this housing type has become so popular in Toronto.
Mid-rise buildings tend to attract more end-users because of their boutique scale. That is, they attract people who plan to move into the building once it is built, as opposed to buyers who plan to rent out their unit. We are certainly seeing this with purchasers at Junction House.
Because of their generally smaller scale and because they are often built in mature neighborhoods with few opportunities for new construction, supply of new mid-rise housing also tends to be limited. That bodes well for future price appreciation.
Here's a quote from Shaun Hildebrand (President of Urbanation), taken from the above Globe article. (Sorry, it's behind a paywall.)