Science fiction covers follow visual conventions so consistent that experienced readers can sort a shelf without touching a book. The conventions exist to solve a specific commercial problem.
The cover has to answer one question quickly
Science fiction is not a single audience. A reader who wants rigorous orbital mechanics and a reader who wants a galactic empire romance are looking for very different books under the same category heading.
Sorting them by description is slow, and a browsing reader gives any individual book a second or two at most before moving on.
The cover therefore carries the classification. It exists to route each book to the readers most likely to finish it and to warn everyone else away.
The visual vocabulary is learned, not natural
Hard science fiction leans on precise hardware, clean lines and astronomical scale, signalling accuracy and restraint.
Military science fiction favours figures in armour and a compositional style borrowed from thriller publishing, because it is competing for readers who also read thrillers.
Literary or speculative work often abandons spacecraft altogether in favour of typography and abstraction, which is itself a signal: this book expects to be reviewed rather than shelved by subgenre.
Designers work from comparable titles
Cover briefs are built by reference to books that have already sold to the intended audience. The designer is told which shelf the book must appear to belong on.
This produces deliberate family resemblance. A cover that looks unlike anything else in its subgenre is failing at the job it was commissioned to do.
It also explains why conventions shift in waves, as one successful look is imitated across a category until it saturates and something replaces it.
Format changes the constraints
A cover now has to work as a thumbnail on a screen, which favours strong shapes, high contrast and legible titles over detailed painted scenes.
Audio and digital editions sometimes carry different artwork from print for exactly this reason, since the same image performs differently at different sizes.
When the signal is wrong
A mismatch between cover and content is expensive. Readers arrive expecting one kind of book, feel misled, and say so in reviews that reach the next set of browsers.
Publishers occasionally repackage a title after a poor start, and a book that failed under one cover can sell steadily under another with no change to the text.