Byran Singer's "name" wasn't as big as Nolan's "name" is now when Returns was released, maybe back in the late 90's towards X-Men's release when he did The Usual Suspects but in 2006
. X-MEN films also aren't regarded as highly as the The Dark Knight or Inception. Also superhero movies weren't as popular or as mainstream as they are right now, they're even bigger than they were before 2008, after The Dark Knight and Iron Man crushed buildings and The Avengers in 2012, 4 superhero movies have recently made over $1B, one is the third highest grossing film of all time another is the 5th. If Superman Returns was released today, it would have made more money that it did back in the day. I would say it flopped because it was a bad film, wasn't enjoyable, had terrible word of mouth and looked outdated. It didn't look like a $200M film. The character also needed to be reinvented, look at how Nolan redefined, modernised and distanced his Batman from the one's before. Yet Superman Returns tried to be like the outdated original Donnor films.