A huge gap between the rich and the poor, invaders from other places, emperors’ deaths and soldiers who were lazy in doing were duty were some of the reasons that cause the Roman Republic’s fall.
The Western part of Rome’s empire fell because fell because the military meddled a lot in politics and being a member of the political office was seen by people as an accountability. Revolutions, invasions by Christianity and Barbarians, and civil wars also contributed to the Roman Empire’s dissolution. The Eastern empire lasted for several centuries as they had a lot more resource compared to the Western empire.