Per the NYT:
"But the government was so slow in issuing specifications that the firm did not start writing software code until this spring, according to people familiar with the process."
Source: http://www.nytimes.com/2013/10/13/us/politics/from-the-start... (October 12)
And from today (Oct 21): http://www.nytimes.com/2013/10/21/us/insurance-site-seen-nee...
In a previous life I worked on similar software development projects at the US State level. This project was several orders of magnitude larger in scope and in complexity than what I worked on. Assuming a generous start date for Spring - say March 1 - this boils down to ~7 months to design and develop to the Oct 1 hard deadline. Nearly impossible in my opinion.
I'm not discounting that there couldn't be issues else where in whole process. These would include: communication overhead between all the various agencies and contractors, integration nightmares, and not enough skilled folks at Medicare to manage all of this. But having the requirements delivered several months before roll-out seems to me a recipe for disaster.