For the most part, the cuisine of any given region is going to feature things that are actually available locally in that region. When regional cuisines were being made originally, people had to rely on things that they could get from local suppliers. They could not bring foods in from a long way away. This is why freshwater fish are more prevalent than seafood in Midwestern cuisine.
The Midwest is not particularly near to any body of salt water. Much of the Midwest is near to the Great Lakes, but these lakes are fresh and will not have seafood in them. This means that it would have been very hard for Midwesterners a century or more ago to have access to seafood. Therefore, it did not become a major part of their cuisine. By contrast, it makes a lot of sense that freshwater fish would become part of Midwestern cuisine. The Great Lakes, along with many other lakes and rivers meant that there would be a steady supply of freshwater fish. Because these fish were easily available, they became part of traditional Midwestern food.