They did. [1][2] According to Brett Cannon who made the decision it came down to three things: GitHub has a built in social network, GitLab has no killer feature over GitHub, and Guido the beloved BDFL prefers GitHub.

[1] https://snarky.ca/the-history-behind-the-decision-to-move-py... [2] https://news.ycombinator.com/item?id=13611387