Here's why:
* Collective nouns refer to a group of individuals considered as a single unit.
* "Citizens" represents a group of people who live in a particular city, state, or country.
Examples:
* "The citizens of the town gathered in the square." (Here, "citizens" refers to the entire group of people living in the town.)
* "The citizens voted overwhelmingly in favor of the new law." (Again, "citizens" represents all the people participating in the vote.)
While "citizens" is a collective noun, it can sometimes be used as a plural noun to refer to individual citizens:
* "The citizens were interviewed about their opinions on the matter." (Here, "citizens" refers to individual people, not the collective group.)
Therefore, the usage context determines whether "citizens" is functioning as a collective noun or a plural noun.