Here's why:
* Collective nouns refer to a group of people or things.
* When "convention" refers to a gathering of people, such as a political convention or a business convention, it acts as a collective noun representing the group of attendees.
Example:
* The convention voted on a new president. (Here, "convention" represents the group of people attending the convention.)
However, "convention" can also be used in a more general sense, referring to a tradition or custom. In this case, it's not a collective noun.
Example:
* It is against convention to wear white to a funeral. (Here, "convention" refers to the tradition or custom of not wearing white to a funeral.)
So, whether "convention" is a collective noun depends on the context and its meaning in the sentence.