Honestly, I think it is because the Scala community was/is/can be so pedantic and bickering and often combative and toxic that it lost all momentum and appeal.

Nobody wants that culture in their workplace. Also the dysfunction is visible in the technical fragmentation of the ecosystem

Scala is way superb for concurrency than what Go or Kotlin have to offer. But it is a complex language with a high learning curve.

> Why do people always mention kotlin

It's backed by $$$ that have promoted it that way - a "better" Java.