REPL-driven development keeps me in the flow. I can write (and test) multiple different approaches to a problem in J or K before I can finish typing the first approach in C++. And that's not even counting time and context switching costs of compiling and running the C++ code.

Lisps and Smalltalk also have repl-oriented development, but they're verbose. Even tab-selecting a Copilot generated autocomplete takes more keystrokes than some array language programs.

The real-time trading systems are in C++, while the data analysis, which happens offline, is either done in Q, R or Python (pandas, polars, etc.)

I never worked with them but I assume it's the same reason cpp guys end up doing some in python, prototyping is easier with instant/short feedback. Later you solidify the solution in statically typed languages.