Real estate is playing within the boundaries city staff set for it. If you want midrises in a rapidly growing city, they have to be actually buildable everywhere. Otherwise if a developer knows they could sell 700 units on a site, instead of the 200 suggested by staff why would they not push for greater heights? Midrise construction is cheaper on a per unit basis, but doing so on the very small % of developable land is simply leaving money on the table from the developers perspective.
The goal of mid rises on avenues was a dream of landscape architects masquerading as City Planners, not a serious economic plan to achieve a housing surplus in Toronto.