Unlike the other Hispanic American countries, Mexico's independence was secured by conservatives. Spain had actually all but defeated the Mexican rebels, and only lost the country because Agustín de Iturbide switched sides in the war. He went from supporting Spain to fighting against it because he felt that an independent Mexico was the only protection against liberalism eminating from Spain (due to the 1812 constitution).
Say Iturbide doesn't switch sides.
How does Spain govern New Spain with all of its possessions in South America gone? How does the U.S. react to being sandwiched between 2 European colonies, blocking its expansion?