Scaling a website of that magnitude, with millions of users from the git-go is unheard of. Even if Google, Facebook, and Twitter engineers developed it it would have still failed to launch smoothly.
Generally, rolling out large scale websites is done in stages. When Gmail launched in 2004, it required an invitation so Google could control its growth. Facebook originally rolled out only to college students at select universities. Twitter, on the other hand, had no way to control its growth and it frequently went down (AKA: the Fail whale).
Since the world's best web engineers wouldn't have been able to launch a website like Obamacare, what made government contractors think they could do it?