i think it's important to codify this answer as much as possible - what everyone else has said seems to be correct to some extent. So to tidy up the thesis:

• Cucking is big right now because of porn. This reasoning works for any of the three participants; a woman is likely to be introduced to the concept through porn, and decide she likes the idea, her partner will be taking part because he enjoys being cucked, and the "bull" will be taking part because he enjoys cucking, from seeing it in porn.
• Porn is more popular with men than women. Women genuinely just aren't as drawn to porn as men; and when they are, they're drawn to stories; and if they weren't, the selection of visual porn for straight women is crap.

I believe those two are the important things.

Other stuff mentioned in this thread, i'll add my own thoughts where i feel like it:

• Specifically sexualisation of black men drives the biggest part of this. [drawing from @Molag_Baller@lemmynsfw.com 's comment] You might also see older couples invite a young man in "as a treat." But I don't think there's any concept of a "more nubile/feminine/sexy" demographic of woman who will "steal your man." We're getting there with how latinas are talked about on the internet so let's revisit this in 5 years.
• Part of the allure is the "risk" to women, e.g pregnancy [drawing from @AgentOrangesicle@lemmy.world 's comment] - I'd also add "going particularly hard, taking a larger phallus than usual" to the list. Some people seem to be turned on by the risk of STDs, which is just one of a few reasons i really don't like cucking, myself.
• To some extent, porn is "designed" - porn is primarily distributed and produced by big companies which stick it the same old tricks, and even if you watch old porn it has the exact same tropes, set-ups, fundamental storylines. You might identify this as a propoganda campaign, 'specially as it comes off as so repetetive. [drawn from @deezcashews@fedinsfw.app 's comment] - I agree, and think it's a reasonable assumption to say that porn exists to demoralise us. But if that doesn't make sense to you, you can also think of it as a business doing the same stuff because it's less creative work for them; they have no idea if cuckqueening videos will ever sell as much as cuck-king-ing videos. Or maybe they already see that they don't perform as well.

Regarding OP's own comment: It's true that men are traditionally more possessive of their spouse, but interestingly, the word "cuckhold" is medieval in origin, and is targeted at men. Cuckqueen is a new word. I think this indicates that men are prone to enjoying "sharing" their wives, but christian culture came into conflict with that and made it more taboo than it would have been before. A writer I've read many books by is Ken Follet and he explores the idea of medieval cuckoldry in his books about Kingsbridge.