Back then, they had different attitudes than we do today about intellectual property. Nowadays, the nerds are in the office and want authenticity and faithfulness to the original. Back then, "auteurs" wanted to put their own stamp on things, to be different and unique and not respect the source material at all. Hence you get Jan Peters (Barbra Streisand's hairdresser), who wanted to do a Superman movie where Superman never wears his uniform, doesn't use his powers, and is angry and destroys things. Basically, completely nothing like actual Superman. (watch this for an entertaining account of this: https://www.youtube.com/watch?v=Wo2KB1dEDdk&t=5s)

That's one explanation. The other one is that they wanted to make the movie based on the book, not based on the 1939 movie.

I think both thoughts went into their decisions.