Why is the Colosseum in Rome damaged?

There are three reasons why the Colosseum is damaged. 

First, it was built very long ago under the leadership of the Flavians. It was started around 70-72 and was completed nearly 10 years later. So, from a historical point of view, there is natural wear and tear. The natural elements for the last two thousand years damaged it. We cannot underestimate this point. 

Second, there were also other natural disasters such as lightning strikes and earthquakes. 

Finally, because the Roman Empire fell and people were building new projects, often times people used ancient sites as quarries. This is what happened to the Colosseum. Many famous buildings were built from the Colosseum. For example, the cathedrals of St. Peter and St. John Lateran were partially built from the Colosseum. Also defense fortification around the Tiber were also built from the remains of the Colosseum.