I think there is no single answer. Some people hear at a conference that Jupyter is "the thing to use", so they start using it for everything while they would be much better off with .py files.
Some use it because the interface was copied form Mathematica, where it makes much more sense than for Python.
These are the serious users.
There are a lot of non-serious users who have discovered that presenting a mixture of code snippets and text that looks good is enough, certainly for PyCon style talks. These users depend on the tool for their fake careers, so they promote it wherever possible, and the popularity increases even more.
Some use it because the interface was copied form Mathematica, where it makes much more sense than for Python.
These are the serious users.
There are a lot of non-serious users who have discovered that presenting a mixture of code snippets and text that looks good is enough, certainly for PyCon style talks. These users depend on the tool for their fake careers, so they promote it wherever possible, and the popularity increases even more.