I've lived in outstate Minnesota and no one called the University of Minnesota (Twin Cities) as "The U." If you are affiliated with the school as student/staff/faculty/fan, you probably call it "The U." But people in outstate MN that I've been around do not call it "The U."
And in other places I have lived and been to, people will shorten their school name but generally, don't call it "The U." In Seattle, it's "U-dub." In Charlottesville, its "UVA." In Columbia, its "USC." In Tuscaloosa, it's "UA." In Lexington, its "UK" and NOT "KU." So nationwide is not correct because most states have another major university within its borders.