Well, this is easy to explain.
The main explanation is that above the line is where it’s quite a bit colder, and more people spend more of their leisure time inside where art of some sort is more likely to get created.
Below the line, there are nearly year-round outside activities that people partake in, thus spending their leisure time in other ways. In other words, when it’s too damn cold to go out 9 months out of the year more art gets created.
(Also, the states above the line tend to be both richer and younger for the most part, so demographics also plays a role.)