The new Superman movie is trying to break the connection to the old ones. If the goal is to make something so it is as if nothing previous happened, then you don't use the same music. You just can't have a completely clean break and fresh start when you leave 1 foot rooted in the past.
I get that people adore the Williams theme - I do too. But as someone who has enjoyed Superman in all media, its not the "Superman theme" to me, its the "Chris Reeve Superman theme".
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