If you want the English Teacher’s analysis, BillyC, it’s because
‘cocksucking’ has too many hard sounding consonants - ‘c’, ‘k’, ‘g’ - and only has one sibilant ‘s’ in its spelling.
‘Fellatio’ by contrast starts with a soft fricative ‘f’, with the subliminal message of air being forced through a constricted passage in the mouth, the one hard sounding consonant ‘t’ is softened by the ‘i’ and ‘o’ to produce a ‘sh’ sound which is reminiscent of saliva slurping and finally ends with the vowel ‘o’, which has subliminal connotations with a mouth around a cock, as well as the sound of satisfaction - ‘Oohhh’ - at the end of a great orgasm!
There’s also the fact that the word ‘fellatio’ is from Latin which is a classical language. Academics in the 18thC decided to standardise ‘proper’ English and used such classical words (and spellings). Therefore, the language of the uneducated and Chaucer, was considered vulgar and quickly fell out of general usage. Educated people ‘made love’. Common people ‘fucked’: Oh no!, I’ve used a bad word. Shock horror


.
Fellatio, on the other hand, was generally considered to be a quietly whispered ‘kink’ and not something that gentle folk practised or talked about in polite society. It was certainly not something that a lady would either know about or do. However, cocksucking was for the unwashed masses who couldn’t appreciate the finer and less noisy aspects of sex. They also didn’t care who did what to whom, so long as the had their bags emptied into the nearest convenient hole.
Finally, anything that is (allegedly) the pleasure of a few - fellatio - takes on a completely new and clandestine connotation of being ‘dirty’ or as I prefer to call it ‘kinky’ as the word gradually gains traction and slips down the social order.
So there you have it. Call it what you will, it’s still great.