Of course, it is impossible to know for sure why certain political trends happen. However, we can speculate that there are at least two reasons for this.
First, there is no necessary connection between cultural and intellectual innovations and economic liberalism. Just because culture is changing does not mean that people will want the government to intervene more in the economy.
Second, the cultural changes were occurring among the young and among certain segments of the urban population. These were the people who were listening to jazz and being flappers. Meanwhile, the bulk of political power was held by older and more rural people. This allowed government to remain much more conservative even as some of the population experienced major social changes.