Because your source was probably the Bible.

If you want real history, read works on ancient history. A good starting point for you would be The Babylonians by E. Wallis Budge. The ancient cultures of Mesopotamia (Assyria, Chaldea, Sumer, Babylon, and Persia) were complex civilizations with rich cultures. They are a fascinating topic. But you won't get their true stories from Biblical history. The writers of the Old Testament always cast older ancient cultures in a bad light because they were polytheistic and more powerful. Essentially its a biased document.

There are other examples. History does repeat itself.

Every high civilization goes through a period of decadence, of decay. The first Middle East cultures, then Egypt, Greece and Rome, even later Western Civilization experienced the Dark Ages until the Renaissance came along. So it would be true to say that Babylon at that period in its history, after having grown rich and successful, was decadent to a point. But to call it sinful is putting an unduly harsh, not to mention misleading, label on it. It also unnecessarily gives it religious overtones.

You can trust every single detail in the Bible. Everything that it talks about is completely reliable. Babylon was sinful because they were not Gods chosen people and they did not follow Gods laws. You can read more about that in the Bible were it talks about the tower of Babel.