JSON is much more static and predictable for configuration. Config through a real programming language was a mistake that many tools made unfortunately. Static configuration has several nice properties including cacheability and simplicity that cannot be guaranteed in a full programming language. Just look at webpack.config.js for an example of the opposite.
All configuration is static. The advantage of using JS (or a language file) is that you run it to generate the appropriate config for that moment and environment.
That need won't ever go away, so not supporting means we're back to users generating their own "static" config files in a separate process. Nothing has been saved, only made more complicated.
That's all totally awesome and cool, right up until the moment you want to add a comment to your configuration. Because you know, it's configuration, and that requires comments.
Quite a few tools went from just JSON configuration files to supporting both .js and .json.
Regarding caching, perhaps I'm mistaken but what's the issue there? Two JSON documents can be equivalent with different bytes so you need to re-parse the configuration file fairly often anyway. So what's the difference with just re-evaluating a .js configuration file and caching the output? If someone wants to put a `if (random() > 0.1)` statement in their config then that's their problem.
You cannot statically cache the results of a .js file - it might include objects or functions that cannot be serialized, and might cause side effects that parcel cannot know about. If we supported .js config files, we would have to intentionally limit what you can return to basically just JSON, so you wouldn’t get much benefit from it and it would be hard to enforce.