Objective-sounding reasons: Scala has got a large library ecosystem, it's demonstrated an ability to retain backwards compatibility while evolving the language (Kotlin may yet get this right (well, no it won't, but that's getting into a more technical argument) but many languages don't, and Kotlin simply hasn't been around long enough to see whether it's any good at handling change).

Developer reasons: Scala is a much more elegant language. Failure reporting in Kotlin completely sucks (in some ways it's worse than Java since they've removed checked exceptions and not offered any substitute - they make a lot of fuss about handling null/options but all that completely breaks down as soon as you want to actually report a reason why something failed). Typeclasses with automatic derivation are really nice. In a big application I might like to be able to do FRP stuff or async pipelines; not only does Kotlin not have an equivalent to fs2 but it's impossible to write one in the language. Every time a new thing comes along Kotlin adds it into the language as a new, different feature because they're allergic to offering general features you could use to implement things yourself (e.g. async/await).