If you zoom out historically, armor changes when the dominant casualty mechanism changes and when doctrine accepts a different mobility/coverage trade. In antiquity it was spear/arrow vs. formation fighting; in WWI it was fragments and machine-gun fire driving helmets and limited torso protection; in WWII and Korea, flak vests for aircrew and some ground troops re-emerged because artillery fragments were (and remain) a primary killer.
The big modern correction is that the post‑2001 era made everyone think “rifle plates everywhere” because of close-range small-arms in Iraq/Afghanistan. But historically, fragmentation is the constant—artillery, mortars, grenades—so militaries keep cycling back toward scalable soft armor and better coverage. Add to that the return of peer/near‑peer planning (lots of indirect fire) and you’ll see pressure for neck/groin/side protection again, even if it’s mission-dependent.
Another trend is doctrinal: decentralization and dismounted operations pushed carriers to be modular, adjustable, and compatible with comms and load carriage. The armor itself is only half the story; the “system” (helmet, earpro, comms, pouches, hydration, IFAK placement) is what changes with doctrine. For sources, this pattern is well documented in standard studies on casualty causes and protective equipment (e.g., Dupuy Institute summaries, historical medical corps reviews, and NATO/US Army protective equipment modernization reports).