You are probably under the impression that MathML is a good idea. MathML is more than an idea. It is Byzantinely complex, unintuitive, unesthetic, highly undocumented, it requires complex software support, etcetera.
Nontrivial problems can have nontrivial solutions.
This judgement is based on experience with all three, and not heuristics. Mathematica and Maple are both proprietary solutions. MathML is not. Arbitrarily complex Mathematica or Maple programs can be written and so can MathML documents. All that software becomes inaccessible in MathML - needs to be retranslated again etc.