It's easy....the problem with the federal site is that everything is bigger. They have to handle the user load from all of the states that decided not to do their own thing. They have to maintain connectivity and exchange data with the insurance companies and plans for every one of those states. They have to interface with their own back-end systems for subsidy eligibility and everything else for all of those states. None of that is easy to coordinate or implement.
In Kentucky, you have a total population of less than 5 million and according to the state, less than 700k are uninsured. (http://insurance.ky.gov/Static_Info.aspx?Static_ID=119&Div_id=16). That's not a huge number of potential hits. A large percentage of the uninsured are eligible for Medicaid under the expansion, which makes the processing even simpler. You limit the number of users, the number of private insurers, and the number of potential plans by doing this at a single state level. The same would not be true for CA, TX, or probably even FL or NY, where there are simply more people and more players.
I believe the states that didn't do their own Medicaid expansion or their own website dumped into the federal government's lap something that is simply too big to manage. I know a lot of money was spent, but can you imagine trying to get all of that data to work back and forth with all of those players? If one of the insurers didn't play ball, did the federal site just kick them out of the exchange? If one of the states waited until the last possible minute to say no to doing their own, what position does it put the DHHS? What about the infrastructure and the development teams?
I hear comparisons on TV to Facebook or other mega websites. The comparison is wrong. Every major website I can think of started as something small and built up to what they are now. A dorm room, a garage, or someone's basement, up to a bajillion dollar a year giant. You don't just set a date for a website and say, "Have at it." I can't even think of an instance where this user count has ever been dropped onto a single site on its first day.....can you?
Is it possible to make it work? Absolutely. Is it simple? Heck no.