Continuity mistake: When Lois drives to the UN Lacy is not in the car. Half a second later she appears inside.
Continuity mistake: When Superman walks to the UN there's a mounted police man on the right. Shot changes, and he's suddenly on the far left. Shot changes again, and he's now meters ahead, on the left of the UN entrance.
Continuity mistake: Nuclear man flies over the Russian parade and looks to his right. Then stops and stares in front and shoots fire balls at the tank below. Problem is the parade, tank and everything is taking place behind him (a matte painting). Laserdisc and international extended editions).
Continuity mistake: When Nuclear Man approaches the tornado the clouds and tornado move at normal speed except for the shot of Nuclear in front of the tornado, where it moves in slow motion. Later on, before the family exits the house the tornado moves in slo-mo again. (Laserdisc and international extended editions).
Continuity mistake: When Superman flies to save the girl inside the tornado and the family exits the basement there's a truck by the entrance door filled with hay. Half a second later the truck is gone and reappears behind the family, but now it's empty. (Laserdisc and international extended editions).
Continuity mistake: When Lois visits sick Clark his fringe changes style from brushed to messy to messier between shots, back and forth.
Continuity mistake: When Lois' car arrives at the UN the rear window swaps between rolled up or down between shots.
Continuity mistake: When Superman walks towards the UN entrance there's a lot of people between him and the red car. A shot later the people have vanished and him and the kid are standing right next to the car.
Continuity mistake: After flying together, Clark and Lois walk into the apartment. When she says she has "the jeepers" her arm suddenly swaps from lowered to raised.
Continuity mistake: In space Superman saves Lacy and flies away with her next to his right side. A shot later she is now on the left.
Continuity mistake: During the Moon fight, Superman is kicked in the head. He puts his right hand on his head, but a shot later his arm is lowered.
Continuity mistake: After flying with Lois, Superman walks away and is about to crash against the camera. He quickly dodges it by turning sideways. From the opposite angle he is walking straight.
Continuity mistake: When Nuclear Man arrives at the place for the first time Lenny is facing Luthor. A shot later he is facing backwards.
Continuity mistake: When Nuclear Man is created he extends his fingers. Shot changes to a wide angle and they are bent and he is extending them again.
Continuity mistake: When the satellite knocks down the cosmonaut, his helmet differs between the wide shots and the close-ups from smooth and with no details, to a real one, revealing the first one is a dummy.
Continuity mistake: The inside of the Russian spaceship differs between the outside shots as seen through the window, and the ones inside.
Continuity mistake: When the movie begins, the cosmonaut has some sort of big compass attached to his legs that disappears in the wide shots.
Continuity mistake: After Superman is pinned to the ground on the moon, he gets some dust on his suit and his hair is messed up, but in the following shots the dust is gone and his hair is perfectly combed back again.
Continuity mistake: From the aerial angle of the Russian parade, the trucks have already passed the tribune where the politicians are, but in the close-up they're many meters behind. (Laserdisc and international extended editions).
Continuity mistake: Superman flies towards the tornado, where the girl is, and "destroys" the tornado (or whatever it is he does), yet, for unknown reasons, Superman is suddenly seen miles away somewhere else, flying towards the tornado again and, what's even weirder: the girl is now floating on air. (Laserdisc and international extended editions).
Answer: There appears to be multiple reasons. They had creative differences, ultimately resulting in a poorly received movie. Kidder said Reeves, who co-wrote the story, had an inflated ego and clashed with Furie.
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Moreover, Mark Pillow, who played Nuclear Man, claimed that working with Reeve was quite intense.