Let me tell you some things about myself as a reader:
I love romance, and when a good one comes along, I can get really, really invested in it. But in general, I prefer romance as a great spice, not the main dish. Also, I tend to like my male love interests sweet and dorky, but I can be persuaded to the tall, dark and sexy side of the force. Especially if they are secret dorks.
I love creepy stories, but I'm not big on reading about the zombie apocalypse. I think it plays into my general nervousness about pandemics, and it stresses me out to imagine myself in the characters' position. I do, however, like more lighthearted takes on it.
I'm not big on dystopian stories about the all-powerful government. I like a messier take on the post-apocalyptic landscape. Actually, my ideal dystopian story is a mixture of both: a society that's only organized on the surface.
I prefer my mysteries in creepy small towns. Shiny crime thrillers don't grab me as much... except when they do.
I'm not a huge fan of gritty revenge stories, except when they are emotionally driven, have sympathetic protagonists, and explore the characters' morality without getting preachy. Then I love them. (If they're done well.)
There are lots of things I don't like. Except when I do like them.
I can work a bookstore like a pro. I scan the shelves, pick up what looks interesting, and read the summary and first page. If it grabs me, I pick it up. If not, I put it down. I put a ton of books down, even books that people have raved to me about. But depending on what's going on in my head that day, the criteria changes. Random little things can turn me off. Any number of factors go into that split-second choice, and I leave without all kinds of great books.
And these are all published books!
This particular ramble was triggered by reading several posts on various forums along these lines: "X agent said they wanted urban fantasy, but they still rejected me!" I know how frustrating it is to be rejected by an agent you think is perfect for you, but this is what they mean by 'this business is subjective.' Maybe your dystopia went organized when they wanted disorganized. Maybe your MC reminds them of someone they really don't want to be reminded of. Maybe you're writing a genre they love, but not the tropes that made them love it.
So to those of you on those forums: next time you get a rejection from someone who seemed so perfect for you, think about how many books you don't buy when you go to the store. It doesn't mean those are bad books. It just means, for whatever reason, they're not for you.