"Why does Joni get candy, but I do not?"
Translation:Miksi Joni saa karkkia, mutta minä en?
I reckon it's because the English equivalent here is "candy" instead of "a piece of candy" or "a candy" (if the latter is grammatically correct). In other words, accusative case would require a countable object, and this sentence uses "karkki" as a mass noun. It can also be used as a countable noun, though.