What makes Postgres unfit for that use? It doesn't have large licensing fees like SQL Server, and Postgres (just like MySQL) can run on a single node and handle small amounts of data just fine.
Getting to reasonably solid active/passive failover for relatively simple uses is substantially easier to learn from zero for mysql.
And I say this as somebody who defaults to postgres or sqlite over mysql where possible these days just because I expect it to be less of a headache down the road.
The knowledge about running MySQL in production seems to be more widespread. A lot of web-hosting companies offers it. MySQL is the default for some CMS systems and so on. Postgres is powerful and can be used for pretty much everything but it takes more knowledge.
