Probably because they don't see the point of eating food with no nutritional value.
This preconception probably arose from a bit in Vonda McIntyre's novelization of Star Trek IV: The Voyage Home, in which she describes Spock becoming inebriated from eating sucrose. Anyone who has diabetes can tell you what this is like. I've never heard anything in canon about Vulcans and chocolate ProfessorTrek 08:06, August 8, 2011 (UTC)
Chocolate is toxic to dogs, as it can cause theophylline poisoning in them