First of all, I think many projects just waited before dipping their toes in kotlin. Now that java itself is adding many functionalities inspired from other la

First of all, I think many projects just waited before dipping their toes in kotlin. Now that java itself is adding many functionalities inspired from other languages, it's much less appealing. I saw some backend projects at $company, and after 2 or 3 years, the decision was made to only use java on new files, and convert kotlin files to java when they are touched. Why? The kotlin compiler was so much slower that it was noticeable, while javac is still very fast. Another reason is available talent not related to Android. Lastly, its value proposition wasn't high enough to justify a different language, especially with java 17 and later. That being said, even quarkus provides kotlin apis nowadays, so it's not like it doesn't happen.