Opinions on this will vary, but for me Kotlin’s ties to the JVM and associated tooling is like a ball and chain. The language itself is mostly fine (though it has some weird design hangups compared to Swift), but I really do not enjoy the tooling at all. Gradle for example has me banging my head against the wall way too often.
Kotlin put a lot of work into proving itself as something more than just a language for Android development, starting first with backend.
It seems like once KMP became viable Swift is just trying catch back up.