Here's why:
* Collective nouns refer to a group of individuals or things considered as a unit.
* A campaign often involves a group of people working together towards a common goal. For example, "The campaign mobilized a large number of volunteers."
However, it's not always a collective noun. It can also refer to a single, organized effort, like a "marketing campaign" or a "political campaign."
So, it depends on the context.
If you are talking about the group of people involved in the campaign, then "campaign" is a collective noun. If you are referring to the organized effort itself, then it is not.