I think the biggest reason I have found so far is that in any team, there always exist rebellious members who always wanted to tried something new. Like in my team, our code base is completely in Java and the leader and vice-leader have no intention of moving to Kotlin, but I and my colleague, the younger members, both learn Kotlin ourselves and want to tried it. The team leaders are very open-minded, so they allowed us to use in a new project, but for the existing projects, we still MUST use Java. Even seeing mixed codes and the over complicated maven pom files makes my boss uncomfortable.
Another reason is that there are many AI research competitions (GVGAI for example) whose requirement is Java. The organizers are great people but they are also old people who are unwilling to change to a new technology. As participants, I and other students must follow the rule and continue working in Java, then when we graduate and get old, we will see no reason switching to a new technology, then the cycle repeats.
I do not readable Java code, I only need working ones to deal with them. Decompiling from Bytecode is, unfortunately, not working (always full of red errors).