I've been thinking about this a bunch, and I honestly still don't know. Julia SHOULD be very popular, but it isn't.
One issue is that if you ask 10 people why it's not, you get 11 different answers, each of them being confident that their particular problem with the language is the universal reason. Nonetheless, by listening to sufficiently many people's complains, we can draw a picture:
* The latency and memory consumption overhead makes Julia unsuitable for a large group of tasks, including many CLI applications.
* Julia is hard to "deploy" since it can't generate standalone binaries, and it's not preinstalled on most systems.
* Multiple dispatch, it's system of abstract inheritance and lack of static analysis and tooling makes it very difficult to write correct code. It's even hard to read Julia code due to dispatch labyrinths, macros that obfuscate IDEs, and lack of good IDE tooling.
I think these are the major issues of Julia that prevents adoption, apart from simply inertia. Most users bump into points 1 and 2 nearly immediately - only later do they realise point 3.