Here's why it's not a traditional part of speech:
* Not a noun: While it refers to a person, it's not a noun itself. It's more like a label or an abbreviation of "Mister."
* Not an adjective: It doesn't describe a noun.
* Not a verb: It doesn't express an action.
* Not an adverb: It doesn't modify a verb, adjective, or another adverb.
In essence, "Mr." functions as a part of a proper noun phrase. It's used in combination with a name to identify a specific individual.