I think, basically "what happened" was rebar... Reinforced concrete allowed for large, open structures where each floor was kind of a plate held up by a few support columns.

Incidentally this also opened the floor for "open plan" offices with only 1 or 2 bathrooms to serve the whole floor, as opposed to previously where each office suite would frequently have its own plumbing.

This is what makes it so hard today to convert the buildings on the right into housing, despite the fact that many of them have been deployed of their population by WFH.