Here's why:
* Adjectives describe nouns (e.g., "The *blue* car").
* Past participles can be used as adjectives, but they usually describe a state or condition that was caused by a past action (e.g., "The *discovered* treasure").
In the sentence "The *discovered* treasure was hidden," the word "discovered" modifies the noun "treasure" and describes its state as having been found in the past.
Here are some examples of "discovered" used as an adjective:
* The discovered artifact was incredibly valuable.
* She stood in awe of the discovered landscape.
* The team celebrated their discovered breakthrough.