Some people just like to stick to what they know already. If you've been coding for few years in a language, it would be more comfortable for you to keep using it than trying a new one.
And it's not just Java and Kotlin, there are other languages.
For example, Python3 was released in 2008, but there are people who still use and prefer using Python2 till this day.
According to the Tiobe Index even COBOL is ranked higher than Kotlin.
Just because a language is newer doesn't make it better. New programming languages are constantly being developed. However, if the advantages of switching do not outweigh the effort of retraining then it will not be adopted.
I'm from an engineering background and, as we like to say, "If it isn't broken, don't fix it".