Or... perhaps those companies are just able to paper over MySQL's deficiencies at a lower cost than switching to Postgres with a product that's already in produ

Or... perhaps those companies are just able to paper over MySQL's deficiencies at a lower cost than switching to Postgres with a product that's already in production. MySQL is very, very well-documented to be a poor database engine that does some terrible things with your data without warning. There's several good alternatives - Postgres being free, MS SQL Server being a reasonable paid-for option, and there's Oracle and IBM's database systems at the high end. One of the only reasons to develop a new project for MySQL (assuming your developers are at all capable of writing queries for Postgres, which is hopefully the case) is that you know your deployment environment is going to have MySQL. Which is probably the case here - LAMP is still a popular stack among PHP developers, so there's a very good chance that anyone wanting to deploy Ghost already has a MySQL server.