The movie's director, Len Wiseman, has moved on to another project, and New Line has slashed the budget and streamlined the epic storyline. From the Times:
But the studio has now cut the budget, going from a film that would have cost more than $100 million to one that will cost a good chunk less than that. It's also reined in the story, turning it into a more simple, straight-ahead invasion story instead of a sprawling epic. Producers are looking for a new writer to handle all this, but haven't found one yet. And Wiseman, while technically still attached, will likely not be a part of it when all is said and done.

The Times also states that Kate Beckinsale, who is married to Wiseman and was rumored to be cast, is now likely off the project.

