You can count a loaf of bread or a slice of bread, but "bread" by itself is not countable. Same with candy, at least in the U.S. -- you can have a piece of candy, or you can just have candy. I think native English speakers in other countries might give you a different answer about candy, though. Now as far as the reason, I am not sure. Leaves are always countable, and so are biscuits. You can use "cake" either way, though. "I'd like a cake" means you want the whole thing, or you can just ask for one or two slices of cake. Or you could just ask for "some cake." For mass nouns, you can count them if you specify the amount, such as "2 gallons of" gasoline, or "a jar of" dirt. That also works for bread. I hope that's a little helpful; I know it's confusing. Good luck with your studies!

I believe cake is noncount. "Have some cake" not "Have some cakes" Think about it this way: you eat a peice of bread and cake, but you probably eat a whole biscuit. Therefore it's easier to 'count' a biscuit, because it is only one serving. But bread and cake having multiple servings, so they are noncount

English is messy, there often is no reason beyond custom. As Meredith said, it likely stems from reasons like us not eating a whole loaf of bread but only a slice!

Count(able) nouns refer to items that can be counted. Uncount(able) nouns refer to items that cannot be counted. It depends what you would like to say. If you say the word “bread”, by itself, it is an uncountable noun. If we say “a piece of bread”, it can be counted. And if we say “one cake, two cakes, three cakes it is countable.” “Candy” we can say “one candy, two candies, three candies”, so it is a countable noun.