When GitLab started supporting MySQL officially (on the paid version), there was a market reason to that. Big corporations used to by expensive Oracle support licenses.

The demand for that have decreased by a lot. I can speculate that Heroku did a good job making PostgreSQL popular as a jack of all trades solution, and a default one for many FOSS communities. The market just followed.

Also consider that some of the issues are due to how Rails and ActiveRecord expect things to be.