How does it work in the United States? Let's say there's a problem affecting South Carolina. How would South Carolina voters convince Californian politicians about South Carolina problems? If convinced, what actions would Californian politicians take?
Problems that are affecting South Carolina will be spoken about in English. Californians will hear about it here and there. They can be seen as national problems.
Something like that doesn't happen in the EU. Every language is like an entirely different political sphere.
US presidential elections (and the party system) add another reason for politicians to care about what the voters in another state think. If Democrats look bad when handling an issue in South Carolina then it can end up costing them votes in other states too.
