I think you're missing the gap between "could fairly easily be" and "has been".
As a programmer, the REPL and/or notebook interfaces for my favorite languages provide what Soulver offers already, so it isn't as valuable to me. To a non-programmer, Soulver seems like a very pleasant, user-friendly tool.
I'm a programmer, at least occasionally, and Soulver is a pleasant, user-friendly tool to me. When I open Soulver, it's because I want to do some unstructured multistep math work and it's faster and easier to do it in Soulver than a spreadsheet program. Jupyter Notebook could undoubtedly do similar work, but it doesn't read to me like it would be the best tool for the job.
As a programmer, the REPL and/or notebook interfaces for my favorite languages provide what Soulver offers already, so it isn't as valuable to me. To a non-programmer, Soulver seems like a very pleasant, user-friendly tool.