system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Significant Factors that have led to the $1.7 Billion Price Tag for Healthcare.gov.
gpt
This is a really big question that gets very complicated and political.  

Basically, it had to take any number of complicated corporate systems with all fine print, discern what was needed and important, then translate that into a unified marketplace.  It also had to do it perfectly, because any misinformation meant the federal government helped a corporation kill grandma because she didn't know her heart pills weren't covered.  

Insurance companies are betting you $/yr that you'll spend less than that on healthcare this year.  They profit by winning enough bets over a given populace. To help their odds, they will pre-negotiate lower medical costs with larger medical groups and put in exceptions like "We don't cover elective surgeries", then get shitty with the definition of "elective" in the fine print.  Shit like "That cancer surgery is "elective" because you haven't tried drug therapy yet".  

Healthcare.gov needed to read ALL of that fine print, for every level of every policy, filter out the ones that don't 100% meet the 15,000 pages of ACA standards and then post the differences using less than a tweet's worth of characters.  All of this needs to be back checked with teams of lawyers to make sure it's presented in a way that a reasonable person can make an educated choice.  

Then there's the technical side.  The website had to plug into at every participating insurance provider's unique and universally terrible corporate sales databases, transmitting application and payment information perfectly, and again making sure there's no fine-print switcheroos for every policy.  Think of the worst corporate management software you've ever seen.  Then consider a the slickest and most user friendly.  You get to write software that works with ALL OF THEM.  Each interface is it's own special shit-snowflake that you get to automate.  For every policy. For every company.  

Oh, and every state has their own branch of the company, and set of state standards, so your fix for Aetna New Jersey isn't going to work for Aetna Missouri, because that has to flow through Mo.Gov's shit website too AND Aetna MO's database didn't get the update yet so they're still using the system from 2003 which is totally different anyway.  So now your shit-snowflake work just went up by a factor of 50.  Also, not everyone speaks english so everything for all policies, fine print, marketplace summaries, and user interfaces has to be accurately translated into a couple dozen languages.  Don't fuck that up or you're killing grandma again.  

While the backend guys are designing a shit-snowflake shoveler, another team has to program a web interface that grandma can use.  See, for the first time EVERYONE was REQUIRED to have health insurance.  That includes your grandmother who browses family photos by dragging them into word documents because it's the only way she knows how.  It also includes the homeless guy who literally cannot read and was genuinely unaware that computers could be used for anything other than porn.  Your interface needs to be so simple that they can figure it out on their own.  

Oh, and since it's collecting SSN's, addresses, and health histories, it all needs to be super secure and require a secure username/password when logging in.  If you're in the identity stealing business, it's goddamn christmas morning in there.  So the site needs to be easy enough to access that illiterate grandma can get in, but also hard enough that 1337HackerBoi69 can't.  

Then it also needs to be ruggedized against political enemies that want the site to fail.  Obama's been telling everyone Sept. 29th is opening day, so how bad would it suck if the site was hit by a DDOS for the first week? Oh, and any fuckup from the aforementioned issues before will 
definitely be blown way out of proportion, so build some lawsuit budgets into any work you do building the site.  

And all of this needs to be done ideally within a year or two, because we've got a political schedule to keep.  Oh, also, Aetna MO finally got their system updated to the 2009 version so the plugins you hacked together for them stopped working and need to be rebuilt.