I assume you mean the 410 sack of Rome.

The Visigoths were the people doing the sacking. They were fleeing war and wanted land to settle and wealth. Rome refused to give them land so they sacked the city.

The Visigoths were Christians though so the sacking was not as bad as it could have been. They respected Christian sites and wealth, they even returned wealth to a church when they found out it belonged to St Peter. They still raped and murdered and enslaved but not as much as they could have.

Because the sacking was about gaining wealth rather than destroying Rome for the sake of it they didn't need to destroy the architecture so they didn't. Their focus was grabbing what they could and going to find somewhere to settle. When the people around you are taking all the gold you aren't going to bother with tearing things down.