Here's why:
* Collective nouns refer to a group of individuals considered as a single unit.
* Visitors refers to a group of people who are visiting a place.
Example:
* The visitors were impressed by the museum's exhibits. (Here, "visitors" represents the group as a whole.)
However, "visitors" can also be used as a plural noun when referring to individual visitors:
* The visitors all had different interests. (Here, "visitors" refers to multiple individuals.)
So, the usage of "visitors" as a collective noun or a plural noun depends on the context.