Because Metro Airport is owned by Wayne County, only the county political leadership has responsibility for the airport. However, the Detroit metropolitan area has in excess of four million residents, only half of whom are residents of Wayne County. Many people from counties other than Wayne use the airport.
As a hub airport, residents of other states and countries use Metro as well. If decisions regarding customer service are made at the airport in a political context (theoretically in the best interest of county residents), the interest of the vast majority of the airport's users do not receive equal representation.
Privatization would allow the airport management to focus exclusively on the needs of its customers and avoid the parochial politics of the county. A private airport would not be bound by essentially arbitrary political boundaries. It could better serve the interest of all users.
Free of the political constraints imposed on county leadership, more decisions might be based on economic considerations and business principles. The airlines could deal with the airport on a business to business basis in which the best interests of each business involved (and their customers) is served through negotiation rather than political decision-making.