The root cause seems to be the community. Few Kotlin developers use anything but an IDE so there's little motivation to bring tooling to other editors like LSP would
I suspect that they have a lot of bells in whistles in IntelliJ that are not possible over the language server protocol. But yes it from a business perspective it doesn't make much sense to devote a lot of manpower to making competitors more capable.
Microsoft seems to currently be investing a lot of effort in making developing .NET server apps on a competitor's platform more capable, so I think it's not as cut and dried as that. If you develop a "good enough" LSP server for Kotlin, and have the "best of breed" tools in IntelliJ, then you potentially increase the pool of Kotlin developers, which creates network effects making Kotlin development more appealing, and then you see more IntelliJ use. There can be benefits to making your platform play well with your competitors, if you can still monetize the platform.
