Why didn't you do so at that time?

This is why they can find their way around the world.

That's the reason why I couldn't attend the meeting.

That's why I came back so soon.

That is why so many students are absent today.

That is why all the students in the class agreed with him.

That's why he did not go overseas to study.

That's why I have such an extreme hatred for him.

That is why I got up early this morning.

Why don't you have a rest for a while?