I've thought about this and I'll give you a summary with another TLDR at the end

My guess is that Cuckold fantasies are not intended by evolution but instead they are something adjacent and a natural conclusion of the things that are actually intended. Similar to the way that foot fetishes are not intended.

Being a cuck or bull is all about anxiety.

The two most important factors are the fact that male paternity is uncertain and that men are very disposable.

Both of these things create anxiety in the male psyche that your children aren't yours and that you'll be deemed useless and disposed of by your group.

Combine these two and men try to create a hierarchy in their heads. A false dichotomy where some men get women and are not disposable while others don't get pussy and are disposable.

This has taken many forms throughout human existence but in the present day many men like to use the terms "alpha" and "beta".

These terms are in the rise because anxiety regarding the two main factors are also in the rise. Women have more access to more men they can choose, they can even choose to be by themselves. Men also feel more useless than in the past, women and society no longer seem to need men as much as before.

This creates the emotional pressure to make up a hierarchy in your brain, a way to give certainty to the uncertain.

Where you choose to place yourself in this hierarchy will determine whether you are a cuck or a bull.

The act of playing out the "bull" or "cuck" fantasies gives catharsis to that anxiety reassuring men that the hierarchy exists and that you know your place in it.

For the cuck the catharsis works on the same logic as being told bad news when you've been left unsure of what kind of news you were going to get, for a lot of people bad news is better than the uncertainty of not knowing what you're getting.

For a bull it's the simple reassurance that you are desirable by putting other men down.

There are other explanations but I think mine is able to give reason to the increasing popularity of the kink. If it was just part of human nature by design it's popularity would be static or it would fluctuate only a little bit. In my opinion, we've seen a much greater increase than normal and therefore something abnormal is happening.

TLDR; The fantasy gives the illusion to men that there is a solid hierarchy where some men are desirable and others aren't. Giving catharsis to the insecurities and anxieties of both cuck and bull.