It's all on the design. Most plate armor was designed to deflect blows and prevent the opponent to land a "Clean Hit". Swords had to be designed for heavy chopping or stabbing because trying to cut the armor was a practice in futility. Some plate armor also used less metal because they also had to keep in mind logistics; creating solid thick armor was expensive and not practical when you need to armor a hundred knights. Instead the design when from "Too thick to break" to "Too angular to make a solid blow". A lot of people dismiss bucklers for being small shields, but they're one of the more advance ones due to this understanding.