There were many structural problems that Rome had to face in the 1st century B.C. In light of this, let me name a few of them.
First, from a general point of view, power was so diffuse among the higher magistrates of Rome that there were a number of civil wars. Moreover, the armies began to be more loyal to their generals than the state. This greatly weakened Rome and allowed for their to be a transition to an emperor.
Second, there were also many loopholes that the Romans could use to prohibit political business to more forward. For example, the office of the Tribune of the Plebs could be used to halt legislation. But this could be done as well through religion, such as when one magistrate, Bibulus stated that he was watching the skies as a augur to halt legislation. Things like this was commonplace. Here is a quotation from Cicero:
They deny that it is lawful to be conducted with the people, when there is an observation of the heavens. Do you dare to deny that the heavens were being observed on that day as it is said the law as passed in the curiate assembly? Present is a man of singular virtue, firmness, and serious, Marcus Bibulus: I contend that on that day that this consul was observing the heavens.
Third, the Romans just grew too quickly. They did not have the ability to rule such a vast empire.