Also, non-backwards-compatible language changes fragmented the Scala ecosystem. Combined with the general trend of functional programming hype somewhat dying down in recent years.
Scala to Kotlin stories are extremely rare.
Kotlin is almost non-existent in the data processing world (Spark, Flink, Kafka streams...)
Kotlin's presence in the average CRUD web app or more complex SOA doesn't overlap with Scala that much, as someone who enjoyed the paradigms available in Scala would definitely not move to Spring, and frameworks such as Ktor or http4k are pretty niche. I've seen people move to modern Java, Rust (a lot actually), even Go despite the language being at the other end of the spectrum. But Kotlin, not really.
I also talk to pure Java teams, that never considered anything beyond the typical Java/Spring/Hibernate, they won't even have a look at slightly more modern approaches (Quarkus, Panache, build-time annotation processing and native images) despite jumping on new version of Java rather quickly. They couldn't care less about guest JVM languages.
Roughly speaking I believe that developers who enjoy Scala won't go a lesser language, that looks similar on the surface but specifically takes away what makes Scala special. Then people who dislike it will just stick to the most vanilla Java stack, which benefits from being literally one of the most popular choice on Earth.
Of course there are a few people doing Android or KMP, or who think Kotlin is in the right spot on the backend. But usually they aren't coming from Scala.