They are a cultural descendant of GDS, who make GOV.UK amongst other things: https://gds.blog.gov.uk/about/
All of GOV.UK is open source in a similar way: https://github.com/alphagov
(Source: I worked for GDS.)
Because they have the right approach to software development and the services were so far behind and costing so much it couldn't really be objected to as a strategy.
The other big factor was that all 3 major political parties agreed at the time and it wasn't blocked. It was moved to focus on the needs of citizens owned by civil servants rather than the convenience of government and farmed out.
In the US we also have a new in house team of coders, led, I believe by xoogler Matt Cutts that has been doing an excellent job as far as I have heard.
I think the key here is that outsourcing your tech is rarely superior to highering talent in house.
The process seemed fairly open. The government send requests for proposals and contracting companies can bid. There is no trick involved. I found the requirements to be surprisingly well written on a few I had to read.
In fact one of the first things I reported is the banner at the top of the page saying "BETA This is a new service - your feedback will help us to improve it" - but it does nothing - it's just text, and there's no way of giving feedback. I reported that 3+ weeks ago and it's still useless.
How about the very-less-than-perfect UI? If they'd done even the most basic user testing they would have found lots of problems already, and that they didn't do such testing shows they weren't looking for problems (which is not a UI problem, it's a systemic dev/management problem). I even sent them a link on how to do UI testing, of course that's not been acknowledged.
And the UI will happily hide info you need. Or log you out, losing all the info you've entered because you've not hit the Save button. It's a bit pathetic - I'm a DB guy but I know more about front end usability than they do. And I really doubt they have a test suite of any comprehensiveness.
I could go on - I know the staff at the job centers also have had at least one issue (small but avoidable), I was there when it happened.
For a service intended for use by millions of people, it's not acceptable, not even close.
Shall I go on? I can give specifics, and the total time I've been interacting with this system is probably a couple of hours tops, and I've run into so much.