Another thing about a lot of commercial buildings is the floorplate size and layout. Office buildings often don't care if there's a lot of interior spaces without any windows, but people need outside light. So if you've got a massive floorplate it can be kind of a pain chopping it up into good sized units that meet the demand of the residential market in the area. This definitely varies from building to building though.

There's also a lot of work that probably needs to go in to the ventilation and fire code changes. An office building isn't designed for people having ovens and stoves. It also often just assumes its OK to have less isolation between units for the ventilation, or previously entire floors were considered to be one space ventilation-wise but now you might be trying to split it into 2-3 units that require separation. This separation can also complicate things like AC and heat.

The ventilation issue comes up a good bit with a lot of these poorly done conversions. You end up with units that just don't get nearly enough airflow, and all the windows are sealed so its not like one can just open the window to get more air.