The array languages aren't super popular because of the sharp learning curve. They're a lot different than most other languages, and they have a lot of operators that simply don't exist in something like C++.

A few years ago there was an article about K's use in high-frequency trading. I'm not sure about usage of APL and J, though. BQN is still fairly new, so it will take a while to see much production usage.

If you've ever written code using NumPy, Tensorflow, or PyTorch, you're doing array programming. Those libraries are heavily influenced by the array languages, including taking a lot of terminology (rank, etc.). I've personally found that playing with J and BQN helped me understand Tensorflow, and vice versa.