Since the topic was started in English, I'll post in that language.
Well, I was going to write in the email, but now that I'm here, I might as well make my ramblings public.
They are short(er)--haha--this time around. (<--Nope. Yet again ~20min were lost writing this.)
I have a two-fold hypothesis about that sort of phenomenon: the wild guessing part, and the personal part (wild guessing x2).
First, it's possible people can't (or don't want to) handle peak emotional load all the time during reading - because it's too much stress. As in "Woah, this story
exhausted me, I need a break now". Most people still (I presume) treat reading as a leisurely hobby to pass the time/entertain themselves, and if an anthology of short stories is perceived as a package full of nothing but "right in the
feels, dude!" then reading becomes a chore/challenge, not a hobby. As for the "distillate of a great idea", unless the readers of the fictional exploration of said idea are prone to contemplation/reflection, then I think the "5 minute wonder", which has become so prevalent in today's meme-laden social media, is in effect: "WOAH, that was radical!"; five minutes later: "Hey, a political hot take video with cats! So edgy!"
Second, and this is more of a personal perception of mine, and again revolves around reading-as-entertainment-and-nothing-more: people like to "invest" their imagination in a given setting/series/author. Because: a) they don't have to "learn" new things (setting paradigms, style change, thematic highlights, etc) every time when they sit down to read (as it would happen if they regularly change their reading "landscape"), so their brain works less and has "fun" more; and b) because sense of belonging: they can say "I am a fan of X" and identify as part of that group - X's fandom; and consequently find like-minded individuals, have flame/shipping wars (venting!), write fanfiction (self-expression!), and so on. Lastly, reading longer stuff makes you immerse yourself in reading, taking for example a whole afternoon doing just that, while shorter stories tend to "eject" you from the imagination when you finish them, and it takes conscious decision--even if only a minor one--to pick up the next story, but that's time enough for thoughts such as "I still have work to do" to enter the mind meanwhile.
Of course, there is the possibility short stories face some other unknown stigma that makes them unpopular.
Bottom line, reading (despite its decline) is still viewed as entertainment first, anything else second even in this day and age. And you know how it goes when mindless fun is involved: the lowest common denominator wins, and the flashiest "products" thrive.