The frames were too flexible which in turn makes the body flex a lot. To make an all steel roof would require a boxed frame with reinforcements to keep everything from flexing. A roof that could flex without wrinkling the roof panel and creating the associated noises was left in the normal way of building cars at the time and that lasted well into the 30s.
Budd had been stamping out all steel roofs for the Dodge Bros back in the early 20s as well
as stamping steel roofs for rail road cars. Ford Motor Company could have done a lot of things that were done in high end cars of the time frame but he wasn't building a lot of high end cars. That wasn't what he wanted to do. He purchased the Lincoln line to keep Edsel busy so he wouldn't try to keep changing things on the Ford line but eventually, the depression proved that times and designs had to change for a company like Ford to survive. To make a more expensive car in those years before the war would have been industrial suicide.
By 1930, it became apparent that other manufactures were changing body designs as regular as every two years. If Ford wanted to stay competitive then they would have to do the same thing whether Henry wanted to or not. The 1932 models were the first one year only design change Ford made in more modern times. After that, the major changes were every two years. The full steel roof was finally adopted in 1937. The post war era was still heading that direction but for the death of Edsel Ford, they had to continue the model design one more year. They were only ready for the Bonus Built trucks in 1948 so everything else was changed in 1949. According to Bob Gregorie, The design for the 1949 Mercury was the last design that Edsel signed off on before he died in 1943 and it had been re-slated for production in 1948 due to the war. It was originally going to be a design for a full sized Ford and a smaller Ford car was also going to be offered. Henry Ford II and his new advisers stopped that track and had a new car design competition for the new Ford for 1949 and the little car Bob Gregorie designed was given to Ford of France and became the Vedette. I don't think Henry Ford II liked having Bob Gregorie around after his father died. Edsel & Bob worked closely together in the design department and it may have reminded Hank the deuce too much of his late father.