The biggest problem time-travelers have is interference from those farther in the future. Those buttinskis have better technology, better history, and are always able to get there before you. Usually just enough before you so that when you show up, there they are, thumbing their noses, drinking a pina colada and sitting on YOUR pile of treasure. Of course they have the same problem. At some point the people farthest up the future are so good you realize they are manipulating everything, and only getting the scraps of what they allow you. Then you get depressed.
At first tIme travelers only went back to “lost” civilizations. The nations of ancient Africa, Pre-Bronze-Age China, the cities drowned when the Black Sea flooded, the civilizations that lived on Ice-Age continental shelves. (Not Atlantis. Never existed. Duh.) But then someone got greedy, and convinced the Spanish monks to burn all the Mayan records. Suddenly they were fair game to exploit. Those criminals were caught, but it was too late to change things back. One day someone in the future noticed that Early 21st century humans had transferred most of their records to easily destroyable server farms…