Having pride in where a person comes from is great. There are a lot different ways to show that amount of pride, but perhaps the best one is to wear it proudly on the chest. Kansas T-shirts in particular seem to be very popular in that region.
Some people are just fans of the school, which go by the same name. Others want to just show that they have a lot of state pride either within the state, or when they are traveling.
Outside of Kansas, not a lot of people know much about the state since it is not particularly populous compared to others in the United States. A shirt can actually help bring people together who might have ties to the area. That is a pretty enjoyable experience that can be completely random if it actually happens.