This movie was made to ween people off Superman.
One of the dirty little secrets about Hollywood is that they know how powerful films are for a lot of people, so they make these relatively low or moderately budgeted sequels to cash in, but also to get people off of the drug that is the film.
At least that's my theory. Otherwise I can't imagine why they'd make such a couple of bombs like Supes III and Supes IV ... it just makes no sense.