Not at Stripe, but I guess it was picked as a compromise between efficiency and developer productivity. Dynamic languages like Ruby and Python don't perform well in the first area, native languages like C++ not much in the latter. Rust might be better, but it's still harder to learn than Java and there's less experienced developers to hire. That tradeoff usually leads companies to pick one of Java, C# or Go.

Why not Kotlin as it's arguably closer to Ruby?

Maybe Java includes the possibility to use Kotlin? At many teams I know its up to the team to write parts of Java libraries or services within Kotlin since it's mostly an implementation detail. But again - I'm not part of Stripe so I can't talk for them.

Yeah, I would be interested in the logic here. I'd assume Kotlin to be a far more productive language for ex-rubyists than Java.

Perhaps it's that they don't believe in the future of Kotlin outside of mobile?

In my experience it's usually JVM at these companies, not specifically Java, and certainly in recent years more and more teams use Kotlin for backend services.