One would switch to a free JVM when other JVMs change their licenses from free to paid. OpenJ9 was the first free JVM to which I got introduced. Knowing it was based on the work by IBM, known for high performance and low memory footprint, it was a simple choice.

When your jvm vendor is IBM? I don't see other reasons. OpenJ9 had quite good polyglot vm Platform. I am not sure how well it is now compared to the Oracle GraalVM. As a Singular developer you don't really care about your jvm vendor. Maybe if you already have a lot of IBM specific tooling OpenJ9 will be a nice choice.