Bryan Singer says he will "fix" some things from The Last Stand. Maybe if he had made X3 (instead of Superman Returns) he wouldn't have to go back and "fix" things with Days of Future Past.
He said he was going to fix the continuity errors that have plagued the series starting with
X-Men: The Last Stand and perpetuated with
X-Men Origins: Wolverine.
Also, he wanted to make
X-Men 3. Even though it was his decision to leave the project, Tom Rothman and 20th Century Fox were being notoriously difficult in their dealings with him. They were lowballing him with a mediocre offer, which they are known to do, and Singer wasn't happy with what they were offering. You also have to understand that making the first two
X-Men movies weren't easy either. They cut his schedule by six months on the first movie and slashed his budget on the second (he wanted to incorporate elements like Sentinels and Angel, but the budget didn't allow for such things). Of course, Brett Ratner gets cart blanche and a huge $200 million budget for
X-Men: The Last Stand, which is what Singer was originally fighting for.
On the other hand, Warner Bros. was offering him that cart blanche with Superman. Unlimited creative control, the budget he was wanting to work with, and a decent payday. Warner Bros. is also famous for being easy to work with and allowing artists to flourish; that's why Christopher Nolan has made every single movie of his since
Insomnia with them, and that's why such artists like Alfonso Cuaron and David Yates were brought on-board the
Harry Potter franchise. Leaving X-Men for Superman was probably a no-brainer for Singer, especially with his personal connection to the character. If 20th Century Fox were easier to work with, he probably would have stayed to direct
X-Men 3. They obviously learned from their mistakes and made peace with Singer by offering him
X-Men: First Class and now hiring him to direct
Days of Future Past.
Also, as you can tell in the article, Singer had a difficult time watching
X-Men: The Last Stand because of his emotional attachment to that series. Someone showed him a rough cut of the movie on a computer just so he could be okay with watching it at the premiere, which he sneaked into because of his mixed feelings regarding the whole thing.
I have high hopes for Days and loved X2 but I fear he may be heading down the Shyamalan road with 3 flops in a row. He really needs a hit with his declining track record.
Three flops?
Superman Returns wasn't a flop. It was made for $200 million and earned almost $400 worldwide. That's not a flop.
Valkyrie also earned $80 million on a $60 million budget, which isn't bad either and also not considered a flop by any means. I will concede that
Jack the Giant Slayer will probably be a flop, and it hasn't done well at the box office, but that doesn't constitute three flops in a row.