I feel like there are too many sub varieties to truly answer this, like my answer involves the centering of a woman's pleasure, but I don't strictly consider myself a cuck either so grain of salt and all. I do like watching my partner with others, and don't have a drive to swing so much myself but I also don't consider it cheating and don't enjoy the thought of actual cheating, so I am perhaps between worlds when it comes to this. You mentioned wife sharing so I think my perspective fits there.
Stag/Vixen is my preferred dynamic, and for me it is largely because one of my primary fetishes is just simple lust. Society teaches that women can't and shouldn't experience or express lust. So what would be more taboo than a womans lust driving the breaking of other taboos? In my case those taboos manifest in a preference for casual encounters, strangers, dogging, exhibitionism, and similar scenarios. I can't stand racist/bully/humiliation stuff though, and I imagine it is because those taboos either center the cucks taboos, or are about breaking taboos that don't appeal to me.
Even in many of the scenarios that don't appeal to me the expression of lust from the wife is a huge part of the marketing. Many captions are about how much the women like this new arrangement, or can't go back to their husband because it was that good, or about how she never came like this before etc.
On the contrary, men are depicted as always lustful and frequently corruptible. Its so sickeningly default. A huge amount of porn centers that perspective already, and frankly it doesn't do a lot for me anymore. I think that is why cuckquean specific porn is less common. There is significant overlap with scenarios that have been done to death already.
So I think this genre of porn has come into vogue because
- It breaks a primary social taboo
- It caters to a variety of different taboos as subgenres
- The female pleasure centered nature is less common and so more valued.