Here's why:
* Verb: The core meaning of the phrase comes from the verb "try."
* Phrasal: The addition of the preposition "on" changes the meaning of "try" and creates a new, distinct meaning.
Example:
* "I'm going to try on that new dress." (Meaning: to put on clothing to see if it fits)
Other examples of phrasal verbs:
* Turn off the light.
* Look up the answer.
* Pick up the phone.