I don't think there is an exact reason why we use the past participle
to form the perfect tenses. It's just that English works that
way. It's just the definition of what a perfect tense
is! The forms developed throughout history to what they are
today, and this is just how they are in modern English. Most of
the languages of Western Europe (English, French, Spanish, German,
Italian, etc.) all have the same pattern or a very similar pattern for
the perfect tenses.
CJ