Oracle licensing audits scare off plenty of folks.

At a small SaaS we were put off by the difficulty getting OracleDB's dev edition installed and working at all. While MySQL (then independent) and Pg were bare bones in comparison, they were very quick to get started, covered what was needed, and no risk of price spikes or time consuming audits. When pain points were encountered with MySQL and Pg there were plenty of flexible options to add into the mix.

Later I saw a competitor being crushed by licensing for Informix when they didn't need more than ~10% of its features.