Marvel's current problem with films is that so many of their heroes are owned by different studios, which is why Spider-Man won't fight alongside the Avengers. All of the DC heroes are owned by one company, Warner Brothers. Their problem is that they have absolutely no idea how to use superheroes that don't star with Bat. Green Lantern, which was supposed to be the Iron Man-esque kickstarter for a Justice League project tanked. Now the latter project is on life support, and only Superman can save it.
Originally, Man of Steel was meant to be a stand alone project anyways, but the success of the Avengers led to the announcement that a Justice League movie was in production, and that the Snyder project was changed to be connected with it. The studio seems to have gotten cold feet, because Justice League has been put on hold. The reason? They're waiting to make sure that Man of Steel makes the kind of money they need it to in order to justify following through with Justice League.
They certainly have every right to be concerned. Superman Returns wasn't the megahit that the studio was hoping it to be, so Superman is really an unproven property in this day and age. Buzz around the whole production is positive enough that it seems like it will be received well, but you can bet there are some nervous suits behind the whole thing.
Man of Steel is set for a June 14th release.