Money

Wikipedia explains...

Production of Superman IV began in 1986. In his autobiography, Still Me, Reeve described filming the movie:

We were also hampered by budget constraints and cutbacks in all departments. Cannon Films had nearly thirty projects in the works at the time, and Superman IV received no special consideration. For example, Konner and Rosenthal wrote a scene in which Superman lands on 42nd Street and walks down the double yellow lines to the United Nations, where he gives a speech. If that had been a scene in Superman I, we would actually have shot it on 42nd Street. Richard Donner would have choreographed hundreds of pedestrians and vehicles and cut to people gawking out of office windows at the sight of Superman walking down the street like the Pied Piper. Instead, we had to shoot at an industrial park in England in the rain with about a hundred extras, not a car in sight, and a dozen pigeons thrown in for atmosphere. Even if the story had been brilliant, I don't think that we could ever have lived up to the audience's expectations with this approach.

Rosenthal's DVD commentary pointed to this scene as an example of Cannon's budget slashing. According to Rosenthal, Reeve and director Furie begged to be able to film that sequence in New York in front of the real United Nations building because everyone knew how they looked and the Milton Keynes setting looked nothing like them, but Cannon refused. According to Rosenthal, they were "pinching pennies at every step".

Superman's childhood home in Smallville was built on farmland outside Baldock in North Hertfordshire—even though the original set from Superman: The Movie was still standing in Canada.[16][17]