I don't think it's that. They switched to OpenJDK a while ago.
It seems to be more that they don't prioritise keeping up with the Java ecosystem very much, they'd rather work on their own stuff. It's more a cultural issue than anything else IMO. Google has always had a big NIH problem.
Apparently they thought they could get away with it.
And there are still lots of jars on Maven central that are unusable on Android Oreo.
Because even after switching to OpenJDK, they cherry pick APIs from there and ART does not support all JVM bytecode features.