This is because normal execution and async functions form distinct closed Cartesian categories in which the normal execution category is directly embeddable in the async one.

All functions have color (i.e. particular categories in which they can be expressed) but only some languages make it explicit. It's a language design choice, but categories are extremely powerful and applicable beyond just threading. Plus, Java and thread based approaches have to deal with synchronization which is ... Difficult.

(JavaScript restricts itself to monadic categories and more specifically to those expressible via call with continuation essentially)