Answer:
This one is complicated to answer, as there are many factors that helped contribute to the fall of the Roman Republic, some that are even debated today by historians. Firstly, the territory that the Romans had conquered became too large for the Roman Republic to handle effectively, and so there was chaos all over the area. Also, as time went on the people in power (specifically the Senate) were very corrupt and were there for their own self-interest instead of helping improve the Republic. Additionally there were many military generals (like Pompey and Julius Caesar) who wanted supreme power and would fight civil wars to get it. Finally, there were frequent attacks of barbarians, specifically of the Celtic and Gallic tribes. There are a lot more reasons but these are some to name a few. I would suggest reading more about it on ancient.edu.