Aside: I wasn't a big fan of the Mathematica language so I wrote "PyML" a looooong time ago- it turned unevaluated Python expressions into Mathematica, transmitted them via MathLink to be evaluated the Mathematica kernel, and then the results were send back and converted to a Python expression. This was long before sympy. It never went anywhere (I found ways to simply not depend on Mathematica).

But it's a very different use case than Matlab. One of the things I like about python is it isn't too bad for sliding between lispy/functional modes for some things and more lapacky things elsewhere. Matlab has a better syntax for the lapack parts but trying to do anything functional gets really annoying and very, very slow. But still I would say Mathematica has the best pure math syntax for those sorts of things. I don't have any experience with sympy, I should probably check it out.

Unfortunately, Wolfram himself has made a number of decisions that greatly limited its applicability and so it will always be niche.