Brett Cannon's blog post (https://snarky.ca/the-history-behind-the-decision-to-move-py...) has been linked a bit and covers the general areas like the social aspects but your remark about open-source hit what I think is a fairly important point in the section about volunteer time:
“The second lesson I learned was about the dedication of volunteers. When the decision was made to switch to JIRA, one of the key attractions of that platform was that Atlassian was going to be hosting our instance and providing direct support (they were very involved in their proposal). But when the community started to protest over the idea of a closed-source, Java application I publicly said if we could get enough volunteers to manage our own Roundup instance then I would relent to using Roundup. There was actually a decent number of people who stepped forward, so we switched (the FSF offered to help put the call out for volunteers but in the end I didn't take them up on the offer as my own personal call seemed to bring enough volunteers forward). But what ended up happening is nearly none of those volunteers stuck around. At this point we have Ezio Melotti and R. David Murray to thank -- both core developers -- for keeping our issue tracker up an running and Upfront Systems for hosting it. This experience taught me that the people you can really count on are those that put the effort into the proposals themselves and those with a proven track record. While people who come out of nowhere have good intentions, that doesn't guarantee they will actually follow through (which I honestly should have known based on my experience from the Python core sprints at PyCon where people used to regularly come to tackle a big problem, get part way to a solution, swear they will finish when they get home, and then never be heard from again).”
That's not a slight on anyone but just an acknowledgement that there's no shortcut for ops. Moving the core Python repo to GitHub outsources that to a fairly large company's primary business and is staffed accordingly. For a project without major corporate support not having to manage a complex service is a big selling point.
It may also interest you that Kallithea, an open source fork of the GPLv3 release of RhodeCode and member project of the Software Freedom Conservancy, was in the running at some point. From the same blog post:
"... PEP 474 ... Nick proposed moving to Kallithea ... Nick Coghlan subsequently saying he would rather back GitLab over Kallithea (due to maturity issues of the projects) ..."
Afair it was because at the time decision was beeing made it wasn't open source, as it is now.