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.)

“Price growth between the two building types [mid-rise and high-rise] began to converge in 2018, and in Q1-2019, buildings under 12 storeys saw average resale prices per square foot grow 10 per cent year-over-year, compared to 6.5 per cent for buildings of 12 or more storeys,” Mr. Hildebrand said. “We may be now entering back into a period of outperformance of mid-rise buildings as the market is shifting.”