Some things that Wolfram Language does that maxima doesn't: machine learning, image processing, audio processing, time series analysis, user interface construction, build-in data, geo computation, natural language processing....
Maxima is just a computer algebra system- one small part of what Wolfram does.
Are Maxima's CAS algorithms comparable to Mathematica's? I have tried to used Sage and other systems in the past, but Mathematica has always been better at finding solutions for what I tried to do. But I have never used Maxima for real.
Maxima is just a computer algebra system- one small part of what Wolfram does.