Sometimes, you change a person in the role simply to distinguish a different take on the same series. It would be jarring, for example, if Brandon Routh went from the type of Superman movies he made (which were fairly meme-worthy), to something as dark and gritty/shitty as the Snyderverse. Or, you go from Pierce Brosnan in tux-wearing campy Bond movies, to Daniel Craig in the parkour, crane-running Bond movies.
If they're substantially changing the tone, the gravitas of Cavill's Supes would be out of place. I love Cavill as Superman, but you can't hold onto things like this.