Why did the Dothraki stop their campaign of destroying different societies and returning things to a wild state? Their defeat at Qothor did not stop all of them.

It didn’t stop all of them, but it probably caused a huge social/cultural crisis – death in battle against a fellow horseman is honorable, but to be defeated by a non-horserider is a huge shame. 

So why risk a dishonorable death, when the city-dwellers will give you riches for going away?