I agree that it would be very nice to have the items you describe available on the web site from day 1.
But those features, are not, and were not required, for the site to GO LIVE.
You are describing a feature you would LIKE ... the government, initially in setting up this site, had to focus on the REQUIRED elements so that, as of Oct 1st, people could actually sign up. Everything past that is gravy. But if people can't sign up, and pick a plan, the entire thing is a failure.
The number of variables involved in this are so numerous, and have been varying for so long, that trying to get all the way to the point where one gets to have a shopping style experience like you describe was determined to be a RISK to getting the REQUIRED elements ready by OCT 1st. That does not mean such features are not valuable, or even planned, it just means they were not "release defining", they were not "critical" for going live.
Every additional feature in a SW development project is a risk to the scheduled release date.
Now, for some software projects, you just move the dates to accommodate new features that you'd LIKE to include. Some times you can get away with that. In this case, there was NO moving the "go live" date. That date was fixed.
The questions was "why didn't the government do it like this?" ... with "this" being a more shopping like experience. The reason was because that shopping experience, while nice to have (and I'd bet its coming pretty soon) was not required to "go live".