I think there are just certain fundamentals that you're forced to learn when you don't have a language as convenient as Kotlin. In Java, you have to learn to use builders since there are no default/named params. You have to learn how to overload constructors properly. You have to use getters and setters for your methods.
Things like lambdas (which you need for a lot of stuff even early on, like on-click listeners) look really simple to us, but are kind of foreign to newcomers; in Java, it's a lot easier to understand that it's just an anonymous instance of some base type, with an overridden method.
Plus, when you do things procedurally with a loop in Kotlin, the IDE will recommend you autochange it to the functional version, and that would confuse and muddle concepts for newcomers too. I just think that Java is so simple that it puts aside syntactic sugar and the "cleanest" way to do something and focuses on just teaching someone how to get from point A to point B (maybe not in the best way, but in the way that requires them getting intimate with the fundamentals)