It would also be interesting to compare those Spring Kotlin numbers against the versions back when Spring was equally motivated to support Scala and Groovy a decade ago.
In any case it hardly matters when the platform only supports Java out of the box on the JDK, without extra tooling, and anything besides InteliJ is a second class experience.
Intellij is written in Kotlin. It does Kotlin really well. Better than Java even.
InteliJ is partially written in Kotlin, unless they decided to port everything on top of Kotlin/Native.
As for why Kotlin exists at all,
> And while the development tools for Kotlin itself are going to be free and open-source, the support for the enterprise development frameworks and tools will remain part of IntelliJ IDEA Ultimate, the commercial version of the IDE. And of course the framework support will be fully integrated with Kotlin.
https://blog.jetbrains.com/kotlin/2011/08/why-jetbrains-need...
You can see that in the statistics for Java 8 in the link you provided. 37% is still stuck on Java 8. That's quite old. We're talking about projects that have not updated anything in close to ten years. And some even use older versions than that apparently.
That's not because those versions were that good but because some people just are that conservative. Java was always popular with conservative companies like banks. Companies like that are likely not using anything released in the last six years. According to Jrebel, that's about half of the survey. You can read into that what you want. But it's kind of meaningless for new projects.
Kotlin had their 1.0 release only six years ago. Spring boot 2.0 followed two years later and that was the first version to incorporate a lot of Kotlin and add explicit support for it.
It would be more interesting to get some statistics for Spring Boot 2 projects and Kotlin usage. Or even break them down by point release (2.7 is coming out soon) and there should be a version 3 by november as well. I'd expect a jump in Kotlin users with each of those point releases. JDK 17 / Kotlin 1.6 are going to be the minimum supported version for v3.
So, the percentage of users of anything over java 12 vs kotlin users is kind of suggestive: 12% vs 8%. Modern Java seems to be not that much more popular than Kotlin. And of course there's a difference between people that have upgraded their JVM and those that are actually actively using the new language features in Java.