Why are archelogical sites kept in a state of ruin ?

Like when you look at the restored city of Babylon in Iraq that the infamous Saddam Hussein erected over the old ruins, the preservastionists are outraged by it . My question is why ? Why keep such beautiful places like Babylon, the Colosseums of Rome and so many other architectual marvels of the ancient world in a state of ruin ? The Japanese preserved their historical sites as well as the Chinese, making them look new and beautiful like they should be . So why don't we ?