From what I understand, you can convert a lot of buildings that were built before the middle of the century; it's the massive onees that are the issue. Older buildings were designed to let in light and air from the outside. If you break them into apartments, you can get something that's a reasonable size with windows.
But if you try to convert one of those massive square skyscrapers, you run into issues. You could break each floor into a set of massive apartments, but there aren't enough people who can afford them. You can make really long, thin apartments with windows at one end, but most people don't want to live in something that's 10-15 feet wide, a third of a city block long, with windows at one end. Or you can put the apartments around the edges and then do something with the center space; say, put tenant storage space every 3 floors, a gym every 8 floors, a play area every 5 floors, etc. But that raises the cost of the apartments and incurs monthly fees to clean and maintain those areas.