At the beginning of this article I asked, why Rome had kings and how did they lose power?
Monarchy was the default political system in the ancient world. Greece was an exception to the rule, although Sparta had kings. In Rome, as the society developed, economic classes formed, with the wealthy at the top. Perhaps it was an oligarchy or aristocracy that became established. Then, at some point, a leader emerged and became king. This story was common place in antiquity.