All this has been decided for months and the discussion about has been going on for more than a year, so no, no recent outage could have been a factor. PEP 512 mentions why GitHub was choosen over GitLab.
tl;dr -- Devs are more familiar with Github, and Python's creator also prefers it (he modestly agreed to go with whatever the team decided and it appears they very much wanted to grant him the courtesy).
https://www.python.org/dev/peps/pep-0512/