Why don't Jupyter notebooks use .jpr file extension instead of .ipynb?

Because Jupyter originates from the IPython project, hence the ipynb (IPython NoteBook) extension.

IPython initially was a monolithic project that offered the notebook interface and the Python kernel in a single release. When the notebook interface was moved to a separate project, I suppose they kept the file extension for backwards compatibility.

I guess I should have added that I know where the .ipynb originated, and therefor at least part of the reason why Jupyter notebooks (still) use the .ipynb extension, but I'm curious why nobody said, "hey, let's make this extension simpler", or when they did, why it didn't get enough steam to be implemented.

Because what is now Jupyter used to be called iPython, and iPython Note Book ... ipynb