I'm guilty of this. I have a project I'd like to open source and it's currently using a blend of MySQL and Mongo. I plan on moving it over to Postgres before publishing its source.

I've been mostly happy using MySQL for relational things and Mongo where it makes sense, but there is a certain amount of overhead in setting up and configuring multiple different types of datastores. Securely deploying Mongo in particular is a minefield, and expecting other people to be willing to learn the nuances of this stuff seems like more work than just saying "use Postgres".