Because they secretly hate all Jazz, but will only admit to disliking fringe Jazz music that most of their friends won't listen to, because they're afraid that one or more of their inner circle might be really into the stuff?
S'only explanation I can think of.
(Having personally, a few (5+) years ago, been in the situation where I've said Jazz is the spawn of the devil to people I looked up to, only to find they love the stuff, I can't blame them either! The irony? I actually now quite like Jazz... must be an age thing... or a university thing... I'mma go with university...)