Here's why:
* Regular verbs follow a predictable pattern for their past tense and past participle forms (usually adding "-ed").
* Irregular verbs have unique, often unpredictable forms.
Here's the conjugation of "write":
* Present: write
* Past: wrote
* Past Participle: written
As you can see, the past tense and past participle forms ("wrote" and "written") don't follow the regular "-ed" pattern.