c'est pourquoi 英語で:

I'm tired and that's why I'm in such a bad mood

I didn't pass this exam that's why I have to retake it.

I have a cold that's why I didn't go to church.

He wanted to rest that's why he took a day off.

you didn't learn that's why you got worse note.

This is a good locations that's why we want to buy the flat.