Kotlin is practically a no-brainer when you have JVM at your finger tips, versus something like Swift which is comparatively young.

I tried to use Vapor with Swift recently and struggled to get something working because the documentation looked comprehensive, but had a lot of gaps. I ended up throwing it out because I didn't have the time to dig through the source to understand how to do something, when I could use a mature framework in any other language instead.

The promise is there but I'm just not ready to invest. My youthful days of unbounded curiosity are coming to an end and these days I just want to get something done without much faff.