Kotlin is unique in the sense that the language originated from a company whose primary business it is to provide developer tooling and IDEs (intellij). They know a thing or two about what developers like and appreciate in their products.
It shows. This language is very nice to use for programmers. Lot's of clever syntax that minimizes boiler plate. Lot's of clever IDE support. Lot's of convenient features. Good build tooling. And so on. In contrast, Scala was always a bit all over the place. Lot's of different takes on how to do things; not all of them very successful in retrospect, tooling was a bit iffy, and so on.
It's the reason Kotlin got popular with Android developers early on because it provided a massive improvement over plain Java and sort of worked as a drop in replacement and was easy to switch to. As soon as Kotlin became available (pre 1.0), the Android community was all over it because at the time they were stuck with a pretty outdated Java language version. It did not take long for Google to make it the language of choice and start providing lot's of Kotlin focused updates to their SDK.
For the same reason, it got popular with Java backend developers early on. It doesn't really matter what Java backend framework you use; Kotlin is a drop in replacement for Java and the chances are pretty good that your framework of choice already has extensive Kotlin support in the form of Kotlin DSLs, extension functions, etc. That's certainly true for Spring/Spring Boot, Quarkus, and most other mainstream server side Java frameworks you can name.
The comparison with Scala is fair because a lot of Kotlin libraries are inspired by work in the Scala community. Additionally, it seems a lot of former Scala developers are now doing a lot of Kotlin. For better or for worse, it seems to compete pretty effectively with Scala. Either language of course is a pretty big improvement over Java; even with the improvements that Oracle has been trickling in over the last few years.