Well, it's easy.
I believe the Second Amendment is sacrosanct because without it, America is literally not any different, theoretically speaking, than China, North Korea, France, Russia, or any of the many Dictatorships in the Middle East in Africa, at least in the Government's ability to oppress its people with impunity.
Let's suppose that a man named Bob Johnson became President of the United States. Now, Bob thinks that everyone should have to dress the same way, get paid the same amount of money, have to have soldiers living in their homes, and have to install security cameras in their homes.
Unfortunately for Bob, the people are armed to the gills. There's a gun for every person in America, (around a 1:1 ratio) and the military only consists of a few million. Bob cannot force an unconstitutional agenda upon the people because he knows if he tears the Constitution to shreds, he will be forcibly overthrown and his rule will come to an abrupt end.
You may be saying to yourself "Well, the Army has so many tanks, drones, guns, and missiles. No way the People would be able to fight back."
Unfortunately, you forget that a Drone cannot search your home for weapons. Sustaining a war with an armed populace quite literally is nearly impossible because you need physical soldiers and boots on the ground. You need an army to fight a war. Technology cannot do that for you. The only way to subjugate the populace is to take everyone's method of self-preservation away. Forced confiscation, buybacks, whatever it takes.