No that's not the reason - or at least, not for everyone. I created nbdev. I've been coding for over 30 years, and spent over 10 years using R and S-PLUS. I've used Delphi, Visual Studio, Emacs, vim, vscode, and many other editors and IDEs, including many with integrated line-oriented REPLs.
There's a big difference between a line-oriented plain text REPL, and a Mathematica/Jupyter-style notebook REPL, especially when you want to mix and match your charts, image outputs, rich table outputs, interactive JS outputs, and so forth. Also, for experimentation, where you want to go back and change things to see what happens (e.g. very common in data science) I find it much easier and more understandable in a notebook.
I have a video where I show the difference between these styles of working in some detail: https://www.youtube.com/watch?v=9Q6sLbz37gk