Even though Mathematica has been around for decades, Jupyter and its contemporaries have rapidly become a popular medium for collaboration among researchers. Mathematica never really had that property.
Mathematica notebooks are the equivalent of Word documents: they’re passed around by email and executed in local, native software. Jupyter notebooks are often distributed by URLs, and (at least when hosted on a site like Deepnote), they’re editable and remixable by default.
Another factor is likely the licensing terms: Wolfram Research has been famously controlling of Mathematica as a software platform, whereas Jupyter is open-source. It reminds me of the issues surrounding Smalltalk’s adoption.