In addition to the visitors, many of whom are staying over at least one night, there are also hundreds, if not thousands of people working on site and staying for up to a week. These showgrounds, like exhibition centres, need to be located near accommodation and other facilities. Hence they stopped being itinerant, as they once were, and built a fixed site which offers the possibility of income the rest of the year. There's no business case for a site in the country.
It's no accident that the NEC was built next Birmingham airport and its railway station.