What is the most likely reason the powers of the US government are divided

To have a Democratic government and avoid tyranny.

Explanation:

The Separation of Powers is politic model used to govern, which divides government in three main institutions: legislative, executive and judicial. These separation theory was introduce by Montesquieu, who believe that democracy was possible if the power wasn't focus in one institution, one person, because this would mean that a tyranny is stablished.

So not one person has all the power to control the country