It's that the open floor model gained popularity; and the building technique evolved. This allowed huge windows to cast light for everyone in one go, since people were not putting opaque wall everywhere which were restricting whatever light that came through "their" part of the building to reach anyone else. "My window" became "our window". Similarly HVAC system evolved to maintain equal temperature in the whole building; there was no longer a need to bring individual heating/cooling system in every single cell since carefully placing some element would let natural convection do the work.

Removing these walls allowed building to be built and heated/cooled with comparatively less material; something that was positive both economically and - dare I say it - ecologically. Converting these shared spaces into private requires to "undo" all these optimisations and will carry a significant cost. Just like converting a bus into several car is not easy because although the seats are there; you are going to need many engines.