Here's why:
* Verb: It describes an action or state of being. For example, "The meeting coincided with my birthday."
* Noun: A noun names a person, place, thing, or idea. There's no noun form of "coincide".
* Adjective: An adjective modifies a noun or pronoun. There's no adjective form of "coincide".
* Conjunction: A conjunction joins words, phrases, or clauses. There's no conjunction form of "coincide".