The GUI is a big part of what made Mathematica better (for many users) than Macsyma. But it wasn't just the GUI - it was also the document-oriented design where

The GUI is a big part of what made Mathematica better (for many users) than Macsyma. But it wasn't just the GUI - it was also the document-oriented design where you could save the graphs and computational results, as well as the code to generate them, in a single document. Python had a perfectly functional REPL, but Jupyter notebooks were still a huge advancement.