We support a fair number of databases in addition to our own Valentina DB at Paradigma Software, with our tool Valentina Studio. People want to use a platform for any number of reasons that may actually have to do with features - or not. How often have you come across a shop that goes with a single provider only (remember all those Microsoft only businesses)?
I was told about an open source blogging platform called Ghost recently that tries to clone the features of Medium. And surprisingly, I read that dropped support for PostgreSQLthey'd for their 'open' version but are using SQLite3 instead.
Something to keep in mind is that they offer a hosted version of Ghost that runs on top of MySQL. Why not offer that instead of SQLite3? What do you think?