I don't think people understand how small books are as a medium. How they've essentially become niche media as far as most people are concerned. Like the original person said: 5000 copies in a week is enough to be considered one of the best selling in the United States. As an independent author, one of my books actually cracked into the top twenty on Amazon for one of its categories, Dark Comedy, just because it sold like twenty five copies in its opening week. (Warning: Erotica)
But another way to compare it? Look at how wealthy the top performing authors are versus other mediums and how consistent that wealth is. The difference between JK Rowling, who is mostly not wealthy from her books as she's only had the main Harry Potter series as a real success, none of her books afterwards have done anywhere near as well as an author of her infamy should have, and James Patterson, an author who has well over a hundred books credited to him. Of course, James Patterson has his own issues but someone who mostly only goes by books, who has such an incredibly prolific output, with a brand that actually does inspire loyalty and has made him a household name as an author, not just one series... and he's HALF of what Rowling is. The lowest author on that list is equivalent in wealth to the 41st wealthiest director, M Night Shyamalan, and none of the top ten authors make as much as even the 50th richest actor in the world.
Books just don't make money and that is indicative to the fact that people aren't buying them. If they don't buy them, they literally can't consume them. If they can't consume them, they can't be a part of a fandom for them.
Of course, even if books were still big... The Harry Potter fandom really exploded with the movies because there's a LOT, especially about modern fandom, that books are kind of hostile to. Art is a big driving factor for fandom culture, MUCH more than fanfiction, but when it comes to books, many will have covers that never depict the characters. The characters themselves might get a couple paragraphs of description very early into the book but then never again and it's left to interpretation which some artist just won't bother with. Even if they might be interested in that, you don't have scene redraws or the like without them having to come up with the whole thing.
And then there's the time investment problem to reading. As much as I love this medium, VERY few take longer, and none of the others give less without your investment, than books. They take a long time and if you don't have a good theater of the mind, even a good book is going to give you so much compared to a sixty hour video game. Even if we go for something more priced alongside books, books aren't cheap, especially physical copies. A lot of traditionally published books are still pushing about ten dollars, even digitally. For ten dollars on Steam, you can get WILD amounts of value back, making books seem all the harder to get into. Even for trying to read on public transport or the like, there's loooots of mobile games out there for free to kill that time now unlike twenty years ago. All of which matters because right now the economy sucks and most people barely have any money for entertainment so that value proposition is pressing down on a lot of people in a very sad way.
Then there's the final ouroboros that comes actually from fandom culture: Because we are so much more connected, it's so much easier to talk about what you like. To find people who are into it... EXCEPT for books. You are taking a real gamble of being the one crazy person who has read the book if you're just picking up a random work on Amazon. That discourages you from reading anything not already popular, thus narrowing how many books are being talked about, discouraging people even more from trying new things, etc. etc.
It's a rough time to be into writing right now and unfortunately it's pretty questionable when, and if, books as a medium will ever have their day in the spotlight again. Not that they'll ever be gone, books still have the power to tell stories in ways other mediums will simply never be capable of, but they will continue to be niche and that's something people need to remember instead of putting unrealistic expectations on their series.
And if you want to help your favorites series, make sure to leave reviews, word of mouth is still the best form of advertisement in the world and maybe help there be a place for other fans to come to find you, even if it's just a Reddit dedicated to the work. Have a nice day everyone and see you next tale.