Even though Hot Toddys work mainly on the placebo effect, it doesn't quite answer the question of why lemon? The answer is simply, it tastes good. The belief is that the alcohol will help cure your cold and the honey will coat your throat, lessening your cough. Whether it actually does is beside the point. The honey makes the alcohol more palatable, but then the drink is too cloying. If you add an acid, it tastes less sweet and has a bit of a bite. Soda has an incredible amount of sugar in it, and if it wasn't for the citric acid, most people would fit way too sweet. It is the same concept. Why lemons versus oranges, limes or even vinegar? That is a cultural thing. Other places might swap out their acids for a different one, but the only reason is to make the drink more tasty.