It's also explained in the post shared:

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I've heard "why not PostgreSQL?" a few times this week. It was even our original plan in #539! Well, it was a pragmatic choice in two different ways:

The person who volunteered to do the work used SQLite.

I don't want to use solutions that are bigger and more complex than our likely needs. Postgresql is my default for projects, but it does have the added complexity of being a separate service to run, tune, and maintain.