Cassandra got the power of prophecy as a gift from Apollo... but when she spurned him, he declared that — though she was always right — no one would ever believe her.
So that's why it's called Cassandra Truth! Maybe I should read the long bit explaining the trope more often... I tend to read the first few sentences, then skip to the examples.
I can't seem to get my friends to go on TV Tropes, but I guess it's for the better, seeing as TV Tropes Ruins You Life...