Here's why:
* Regular verbs form their past tense and past participle by adding "-ed" to the base form. For example, "walk" becomes "walked" in the past tense.
* Irregular verbs have unique, unpredictable forms for their past tense and past participle. For "wash", the past tense is "washed" and the past participle is "washed".
So while "wash" follows the usual "-ed" pattern for the past participle, its past tense form doesn't. This makes it an irregular verb.