Why is the verb in French an irregular verb?

It seems like you might be asking about why a specific French verb is irregular, not why verbs in French are irregular in general. To help me answer your question accurately, please provide the verb you're interested in.

For example:

* "Why is the verb 'être' (to be) an irregular verb in French?"

Once you tell me the verb, I can explain why it's irregular and how its conjugation differs from regular verbs.