Why did the framers adopt the separation of powers system?

The tower commission, appointed by president reagan in 1987 to investigate how the iran-contra affair had happened, concluded that most of the blame lay with a. president reagan himself, for careless management of his subordinates b. congress, for poorly drawn legislation that failed to define carefully the role of the president's national security council c. the american people, for failing to pay attention to the conduct of international affairs d. the federal courts, for having allowed excessive power to flow to the executive branch