To be honest, it is probably a touch of 'inverse snobbery' on my part. I had a rather ordinary 'working class' upbringing in Watford and I have never knowingly met anyone called Cassandra! It has classical overtones to it - my associations based on the Trojan prophetess familiar to me from 3 of my interests (namely 'Doctor Who' - she is a character in "The Myth Makers" (1965) and Shakespeare - she is a character in "Troilus and Cressida" (quite a bit earlier than 1965) - and ABBA (she is the subject of their song "Cassandra" which was the B-side of "The Day Before You Came" - for me, two of the best things ABBA ever did among many good things!).
I think my attitude towards the name was compounded by the name of Rodney's yuppy-ish girlfriend/fiancee/wife in "Only Fools and Horses" who was the daughter of aspirational parents and clearly signified as distinctly posher than Rodney. The idea being that her parents had given her a bit of a la-di-dah moniker like 'Cassaarndraar' (as Delboy might have put it). As opposed to a proper classy name, like Raquel, for instance.
I ought to say - without irony - absolutely no offence intended to anyone called Cassandra. Or Raquel, for that matter