How and why did Rome make the transition from a republic to an empire?

The main reason the Roman Republic failed is that it was never a democracy. The rich had undue influence in the electoral process. The various generals like Marius, Sulla, Pompey and Caesar were much more popular. There was also increasingly instability, many civil wars. And everywhere outside of Italy was governed as a province, with a Governor imposed by Rome. Most actual inhabitants were excluded from the workings of the Republic.