Rome was replaced as the capital of the late-Western Roman Empire by Ravenna, though it later became relevant again as the residence of the pope and his government. The state of older buildings like the Colosseum did not reflect this, though, as multiple earthquakes, fires, and other natural disasters were endured by the increasingly fragile stonework. According to National Geographic, only a third of the original building remains after years of exposure and a lack of local incentive to repair any damage.
In fact, there were many who were more than willing to loot the Colosseum for its stone or any fittings inside that might have value. Original metals of value found within were often pulled from the walls to be sold or used (via Guide to Rome). It was finally spared total destruction by Pope Benedict XIV in 1749. He declared the site to be holy on account of the early Christian martyrs supposedly killed within its walls, forbidding any further theft of its architecture (via AR-Tour).
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