Here's why:
* Coke is a singular item: You typically buy a can or bottle of coke, not a group of them.
* Collective nouns are for groups: Collective nouns are used to describe groups of things, like a "flock" of birds or a "herd" of cattle.
However, you could use a general collective noun for drinks, like:
* Batch: "A batch of coke was delivered to the store."
* Supply: "The restaurant ran out of its supply of coke."
* Selection: "There was a selection of coke flavors available."
Ultimately, the best term to use depends on the context.