Property developers have to buy up the section of a block to demo(lish) and build something worthwhile. Many of the buildings are individually/family owned, with little incentive to change/think about change or some pride in passing the building down generationally. If you have an annuity (rents) sitting on top of appreciating land value and you're old or a son/daughter (usually living elsewhere collecting a rent cheque), why bother/how can you convince your neighbors to as well? That's why the section around Ainslie and Union I mentioned was able to be converted, they're all business/warehouses, a condo there now was in the 1940 picture an auto repair garage with the same footprint as the condo, but the house next to it still stands as is.