I never understood why Hermione has no trouble at all believing in Vampires, Werewolves, and Unicorns, before she's met one, but has such trouble with the posibility that Snorkacks might exist.
Well, that question is at least easy to answer.
The most obvious main reasin is because she has read about them in books or seen them herself.
The second part is because she would naturally question anyone that stated a 'fact' without offering a shred of doubt or proof as to the veracity of their claim.
The fact that Luna's critters have such unlikely names and behaviours would raise doubt in most reasonable people's minds, so Hermione would conclude that Luna wasn't reasonable about them.
If Luna was less insistent about their existence, Hermione would be more inclined to believe her don't you think?