Why did the three branches separate?

Answer 1

It is not that the three federal branches--the judiciary, the legislative branch, and the executivebranch--were "united" and then "separated". They were envisaged by the Framers of the Constitution as "separate" (the effectuation of a plan of "separation of powers") with "checks and balances" between the three. This forms the basis for the Constitutional doctrine of the "separation of powers".

This American Congressional Presidential system contrasts with the British Westminster system wherein the legislative, executive and judicial branches are "separate" in the U.S. system, whereas the legislative, executive and judicial branches are "fused" in Britain--that is, the prime minister and cabinet ministers are also legislators, and some legislators are also members of the judiciary. The judiciary is headed by a separate Supreme Court in the U.S., rather than having the upper house of the legislature (the British House of Lords) also able to sit as a court (with a narrow exception as when the Senate tries an impeachment by the House).

The judiciary, in the form of the Supreme Court of the United States, can nullify Congressional legislation where it finds it unconstitutional, but the judiciary is not supposed to become "activist", that is, to "legislate from the bench". It is only supposed to defer and adhere to laws enacted by the legislative branch in the form of the Congress, in the rulings the Court hands down.

The legislative enacts laws the constitutional validity of and broad effect of which are defined and interpreted by the judiciary, and executed and enforced by the executive branch. The legislative can only "speak the will of the people" in the form of legislation. It cannot negative lawful Presidential action, except by its control over legislation from which the Presidency derives its authority, and by negativing Presidential nomination of "Officers of the United States" and "inferior Officers" through the Senate's powers of consent, appointing Presidential nominees, as set forth in the Appointments Clause, and in specifying which offices are held by "inferior Officers" which, by the Constitution, can be unilaterally appointed by the President. One other instance where the Congress can negative Presidential action is where it overrides a previous Presidential veto with a two-thirds vote in both Houses of Congress, to enact legislation over Presidential objection.

The executive is, by the Constitution, to "take care to faithfully execute" the laws. It can "tweak" how laws enacted by the Congress are applied through executive-branch administrative rulemaking. The President can unilaterally issue Executive Orders, but these are required to state their authority derived from Acts of Congress and rules promulgated thereunder. The President can be impeached by the House of Representatives, and tried by the Senate, presided over by the Chief Justice of the United States, and removed from office upon conviction of all Articles of Impeachment, for "high crimes and misdemeanors'.