The main reason was because of the way battles used to be fought. In the open. Guerilla tactics didn’t really start to take off until much later, even taking until the revolutionary war to really become a normal part of warfare. Simply put, once you are able to shoot and kill people, in full plate armor, from hundreds of feet away and escape without being seen, combat changed forever.

Being more difficult to spot and hit at a distance became a priority vs flashy protective measures like platemail.