Here's why:
* Noun: A word that names a person, place, thing, or idea.
* Example: "We had a lovely picnic in the park." (Picnic is the thing we did)
* Adjective: A word that describes a noun.
* Example: "We had a lovely picnic in the park." (Lovely describes the picnic)
You can, however, use "picnic" as part of a compound adjective, like "picnic-style" or "picnic-basket."