Question

Why was the coliseum repaired after the fire but not after the earthquake?

Answer 1

The germanic conquerors of the western roman empire didn’t like the gladiatorial games. Maybe they remembered how their ancestors suffered by having to fight in these games and it was therefore seen as a reminder of a time when their people were oppressed. So there wasn’t much point or desire in maintaining the coliseum and it fell to disrepair, which only got worse with the byzantine-gothic wars that were to come.

In the east the smaller amphitheaters and combat games continued a while but pressure from Christianity eventually led to their dissolution. I imagine they might have continued as places to recite religious sermons