Most of the scientific libraries of note originate in academia where MATLAB is effectively free to users. The cross over to Python was well under way by ~2014
Mojo is free, though. MATLAB costing money is a bigger issue than it being closed source. R was too late to the game and catered too much to professional math/stats/datascience people rather than programming generalists. Python (with native code interop) hit the sweet spot for breadth/accessibility to the market and capability.
