It makes perfect sense. Just not to a lot of HN readers.
The average HN reader is approaching this from a perspective of "I am a professional programmer who might occasionally dabble in scientific computing, and therefore I hate this thing because it's not a professional programmer's tool designed by and for professional programmers according to the best practices of professional programmers".
The people who are actually using notebooks, meanwhile, are not professional programmers. They're scientists who increasingly have to do programming as part of their science. And notebooks are a godsend for them. We don't need to drag them all the way into our world; we need to pay attention to what they actually want, need, and find useful, and accept that it's going to differ from what we want, need, and find useful.
2/3 of scientific research cannot be reproduced by other scientists. But tell us more about why scientists should ignore best practices from other fields.
Okay, that was extreme, and if you think I was talking about programming, it's because you have a guilty conscience. ;-) It actually applies to all interesting fields -- programming, engineering, management, classical music composition, etc. Those fields don't even know what their best practices are, and acknowledge that things take too long and can't be managed. No manager would say: "Our programmers have best practices, so the work will be done next week." Why should scientists have such faith?
Meanwhile, do you trust Maxwell's Equations, Darwinian evolution, quantum mechanics, etc.? How did we establish the physical constants to mostly better than 8 digits of precision? Science has somehow figured out how to make progress despite the messy business of research.
For me, it's not that I "have" to do programming, but that physical science has been computation driven since before the 1940s. Programming is how I think and work. With apologies to Richelieu, "programming is too important to be left to the programmers."
"Best practices" are a chimera. The issue at hand isn't about what is "best", but whether or not a software engineer's "good enough" practices are more likely to achieve science's goals than a graduate student's "good enough" practices.
It's also disingenuous to claim that classical music composition doesn't have "best practices" when the field of music theory exists as an explicit manifestation of "best practices" in music. Having gone to a school with a conservatory, I also believe that I know several individuals who would would disagree with your mindset regarding how the creative process can't be managed. Indeed, if creativity, as it relates to musical composition, couldn't be managed most orchestras would be brimming with anger at the number of commissions that weren't finished on time for the concert, and most Hollywood studios and Broadway shows would screech to a halt.
They probably didn't take off to the same extent as Jupyter because they're not free. IIRC MATLAB was quite expensive, particularly if you wanted to do anything specialised.
Notebooks have been in Mathematica for ages and are really powerful and difficult to describe to those who haven't used them. To give an example, I was building a tool and embedded images as variables in a way reminiscent of being an engineer on the USS Enterprise. You can point to a file in Python as a variable, but you can't just copy-paste an image in as a variable last I checked (don't think Jupyter is there yet).
The exact same thing happened with the arrival of the www.
The irony, to me, is that I actually typically argue for the mixing of presentation and content. But to me, notebooks look like an attempt by people to make a WYSIWYG out of JUnit/TestNG/whatever style reports. Only, without the repeatability.
There is also the entire bend where these are taking off in a way that doesn't make sense. Do they do the things you are saying? Well, yeah. But no better than plenty of tools before them. Mathematica and Matlab both had "notebook" like features for a long long time. Complete with optimized libraries. And this is ignoring the interactivity of the old LISP machines. (You can see from my history I have a soft spot for emacs org-mode.)
Jupyter is a lot of things. Bad isn't necessarily one of them, but exceptional isn't, either. Heavily marketed is.