Because with JVM languages, and their "horrid" backwards compatibility (that everyone always loves to bash), I can declare dependencies on libraries written in Java 1.1, and it will JUST WORK.
From what I understand, there's a fairly large chasm between Python 2/3 (such that a lot of people use external libraries to deal with coding cross-compatibile code, and apparently get wrong some-times).
From what I understand, there's a fairly large chasm between Python 2/3 (such that a lot of people use external libraries to deal with coding cross-compatibile code, and apparently get wrong some-times).