Being a government contractor, I sympathized with the companies. I watched their testimony. They were given a set of instructions for the site. They started building the site. Then change orders rolled in. They were being asked to make changes up until 2 days before it went live. They requested clarifications on certain items that they never received an answer on.
As with any situation, when you are hired to hit a target, and the target keeps moving, it's going to be very hard to hit it.
Let's think about probabilities, here. These companies were chosen because they had done this before, successfully. They have experience doing it. How often has HHS done this? Never. If you are going to assign blame for failure, who is most likely to be wrong - the people who have done it, or the noobs?