Allow to go to ruin implies passivity. In fact, many of the ancient structures have been repurposed over the millennia. Deep grooves are visible on many columns of temples in the forum. Tour guides and guidebooks say they're from ropes when Christians tried to pull down pagan houses of worship, but it's much more likely they were caused by ropes used to suspend canvas roofs. People kept using the buildings but possibly lacked the know-how or wherewithal to keep the original structures in good repair. It's likely that a negative attitude towards ancient pagans may have also played a part. Look at Palmyra today and remember that not every society in history has valued artifacts and structures simply for their historical and cultural importance.