Centralization of power in the presidency had been going on for a long time before 9/11, motivated by two things: nuclear weapons and a semi-permanent Congress.

Centralization of power in the presidency had been going on for a long time before 9/11, motivated by two things: nuclear weapons and a semi-permanent Congress. Because of the need to be able to respond quickly during a nuclear attack, much of Congress's oversight in military matters were transferred to the presidency. Also, because congress critters tend to be cowards who want to avoid doing things that would upset the voters in their district, the presidency has been given lots of regulatory power it shouldn't have. Congress critters love giving presents to voters far more than they enjoy slapping them with a bunch of regulation and restrictions. Let the president and the Executive Branch's bureaucracy take that heat. 9/11 gave the government, especially the federal government, much more power than it had before, which means everyone's slice of that power grew, but I'm not convinced that it's all that responsible for the slice for the executive branch having grown to be such a large share as it was already there before the attack.