Because

(1) There was a lot of traffic on the way to the airport.

(2) I arrived late.

(3) I missed my flight.

(2) I arrived late because (1) there was a lot of traffic.

The reason (2) I arrived late was that (1) there was a lot of traffic.

That's the reason why I arrived late.

(3) I missed my flight because (2) I arrived late.

The reason (3) I missed my flight was that (2) I arrived late.

That's the reason why I missed my flight.

So / Because

(1) I missed the last flight of the day.

(2) I had to spend the night at the airport.

I missed the last flight of the day so I had to spend the night at the airport.

I had to spend the night at the airport because I missed the last flight of the day.

Related articles: Audio so / because