Armor has changed to meet the needs and demands of a particular time period's training, technology, and combat styles.
If you were sent to a medieval fantasy world- full of monsters, mounted knights, and bandits- and had to take modern armor with you (no plate armor reproductions, only things used by military forces, police, and civilians), which choices of equipment would you choose and why?
The forces that you would fight would include natural wildlife like bears and wolves, monsters (primarily zombies, skeletons, entities that use energy attacks), and human beings protected with late medieval armor (plate armor, mail, gambeson) armed with weapons appropriate for that era- polearms, shields, swords (either one-handed or two, focused on slashing or stabbing), and bludgeons like simple clubs to maces.
And weapons used by you in a fight would be modern firearms like semi-automatic rifles, handguns, and manually operated longarms such as lever rifles or pump action shotguns, with moderate amounts of training in their use.