True; but for random programmer off the street, these languages impose an additional cognitive load. For a given Java library, most of the example code and documentation will be geared to Java itself, and translating that to "another JVM language" isn't always trivial.

Especially since most programmers out there are not polyglots, work in a big company, and may not even by "programmers" but just "engineer/accountant/business person who programs a bit".