Suppose you write a story that introduces “science” or magic that has to be there for the plot to work. Thus in the Nibelungenlied, Siegfried must help Gunther win Brunhild for the plot to progress. Now, in a contest of strength watched by all and sundry, Siegfried cannot be seen doing it, hence the need for a cloak of invisibility. Star Wars would not be the same story without the force. Star Trek would be quite different without teleportation and warp drive. One issue facing a writer is, when should one try to explain what underpins these devices?
One possibility is, “never”. Thus for centuries, people listed to the Nibelungenlied without worrying a jot about how the cloak worked; it was magic! The Star Trek “science” is slightly different. Teleportation in the sense used there was not really important other than as a means of getting on with the story, so as long as the story was good, who cared? It just removed the need for tiresome issues with shuttles, and it was accepted (or not) without further discussion. Warp drive was something slightly different. Again, that was needed to get the story moving, and to permit the crew to return to Earth, but it has an interesting side issue. According to relativity, or more particularly the representation of space-time in relativity, moving faster than light requires moving through time in a negative direction so you can arrive before you set off. As far as I am aware, this peculiarity was never made use of.
This introduces another reason not to explain: the explanation is just too complicated. Think of going back in time through warp speed. This depends critically on the concept of space-time, without which the equations of general relativity are sufficiently difficult that they beat Einstein (who, at first, thought “space-time” just plain wrong, but he later adopted it because there seemed to be no other way of making progress). The author does not need to get bogged down into that discussion! For example, one view might be that just because the maths are more easily solved using space-time, it does not mean that spacetime is a physical object. Think of the geometry problems you solve by making a construction, or a differential equation using a substitution. Making a construction on a sheet of paper does not make it real on whatever the diagram represents! It just makes it easier to solve the problem. Similarly, modern quantum mechanical problems are addressed through using something called Hilbert space. Nobody I know has suggested that Hilbert space is a real thing, and if it were, it is not really compatible with relativistic space-time. See why you do not want to get involved? Why dig a hole for yourself and lose readers?
Notwithstanding that, there is a problem with ignoring it. Thinking of warp drive, you can either do what Star Trek did and use it as a way of getting from A to B to shorten travel times. Now you are incompatible with relativity, so no explanation is a good idea, but you consign the concept to the “convenient” and turn your stories into the “ordinary”. Be careful, or all you end up with is a space western, and the author has lost a huge number of possible plots!
Take another reason. Think of the force in Star Wars. I remember watching the first three movies, and I, along with everyone else I know, accepted that, in these movies anyway, there was something called the force that a very few could access after a lot of training. I did not care what caused it. But then, a number of years later, Lucas made three more movies, and explained the origin of the force. In my opinion, the explanation was ridiculous, and it only detracted from the movie. To summarize what I am suggesting, sometimes it is better not to try to explain something. Details can add to a story, but silly details subtract from it.