Building standards and designs are very different. Commercial buildings are usually built with central bathrooms, so plumbing needs significant changes. Then you have to consider how the floor space is laid out relative to windows. Every apartment needs windows to the outside, so you end up with long, shoe box style apartment layouts that aren't very desirable. And then you have to figure out how to transition from a floor-by-floor HVAC system to one where every apartment controls their own.