Unknown to the majority of the Wasteland, a secret behemoth of terror slept beneath the ground, only to awake with plans to reshape the Wastes in their image.
Before the Great War of 1962, a large contingent of High Ranking American politicians, Military Brass, and Wealthy Businessmen had built a great bunker in the most remote part of Idaho. When the atomic bombs rained down on the country, they retreated to the underground, and hid there for over 97 years. There, while the rest of the Wasteland struggled to survive and rebuild, they continued to advance technologically.
The Bunker Dwellers, as they are known, have access to technology beyond the wildest dreams of a Wastelander. Motor vehicles, filtrated water, rotary aircraft, to name a few. While they have a small population of only 5,000, they can beat entire small armies with their military force.
In my story, they stay in their Bunker from the year 1962-2059, about 97 years. My question is, what is a plausible reason for them staying in their bunker for that long of a time?