What do you think the problem is?

I think the problem is just not enough training on that specific language because it's proprietary. Most useful Mathematica code is on someone's personal computer, not GitHub. They can build up a useful set of training data, some benchmarks, a contest for the AI companies to score high on, because they do love that kind of thing.

But for most internet applications (as opposed to "math" stuff) I would think Python is still a better language choice.