This article on The Hill says that it is so the State Department can focus its precious resources on something more important, that is, on its social media efforts.
Personally, I'm having a hard time with this. After all, the purpose of Web 2.0 was to be able to maintain content in a way such that it doesn't have to be maintained all over the place. With a few inexpensive modifications (unlike some of the multimillion dollar websites that we sometimes hear about with the Federal government) America.gov could be cast in a way that it would be fed by the same social media efforts that the State department is talking about emphasizing, whether those are in Twitter, Facebook, YouTube, etc. Most social media content is portable.
America.gov is too valuable of a property for this to happen. It now looks like the digital version of some of America's blighted urban areas. I hope someone has the sense to bring it back to life where it can prominently integrate with the social media efforts that have replaced it.