I like to think of "Cuck" as the less complicated permutation of "******": It implies a measurable lack of masculinity and confidence, and unlike homosexuality it doesn't carry with it any of the religious or historical baggage. The fact that "Cuck" sounds like a mix of 'cock' and 'fuck' makes it doubly fun to say, too. I don't think we'd have seen it take off as it has in certain circles had it not been simultaneously a specific, masculinity-robbing insult and sounded like two of the naughtier words the English language has been hiding in plan sight for a good long time - it's like a fragment grenade of an insult, being familiar sounding and having a far more specific meaning than, say, Fuccboi ever had.
By the same measure, ****** isn't useful as an insult anymore anyway - let's face it, in the circles that love the term "cuck", the word "fag" is roughly as common a sentence particle in English as 'desu' is in Japanese. South Park opined years ago how language evolves and means different things, using "fag" to mean an annoying jerk, rather than a homosexual. This is the completely natural (if oddly displaced) result of using a taboo word to the point of it no longer creating a reaction in people; if we're all faggots, being a ****** ceases to be an insult.
Which means, eventually, being a cuck will be rendered meaningless as well. Pity. The Game Grumps dedicated half an episode to discussing it, and I figure at this rate it's about a year until something on normalfag TV uses it, rendering its' use as a powerful and exciting new insult completely moot*.
Ask not what the insult of is - ask what the insult of <current year +1> will be!
(*How could I forget? We also saw a dramatic uptick in the term specifically because Chris "Moot" Poole was deemed quite the cuck himself.)