I think it has a lot to do with the weather and people getting together in private spaces. Florida has seen a rise in cases recently but nothing near the multiple of places like Illinois where there were still tighter restrictions, especially compared to Florida. In Florida you can still have comfortable outdoor gatherings and parties to try and be safer (same for California which has also seen some increase).
Basically a lot of the same reason that the flu and colds spread in the winter. The more people are in indoor spaces without good ventilation, the easier it is for any virus to spread.
Bottom line is that viruses exist to spread from host to host. The only way to really slow down the spread without a vaccine is to keep people apart from each other which requires severe restrictions on freedom of movement and gathering. Even many of the "blue" people will only put up with so much.