The reason panel damage turns into a bigger job here more often than on a newer door comes down to what's never been touched. On a door built around 1971, it's common for the panels, hinges, and even the original rollers to still be the factory set - nobody's had a reason to replace parts that weren't broken. That means when one panel takes damage, it's not an isolated weak point in a system of newer parts; it's one worn piece in a system where everything is roughly the same age and the same few decades from fresh.
Add in the severe cold-climate swings this area gets, and steel that's spent fifty winters expanding and contracting is more brittle at the dent site than new steel would be. A panel that might flex and hold on a five-year-old door can crack clean through on a door this old, especially at a weld seam or a rib fold.
It also means the fix has knock-on effects: resetting one panel on original hardware often reveals that the hinge pins are worn oval, not round, which isn't the issue you called about but gets found anyway.