Others have talked about the obvious which is the type of Armour they wear. But my thoughts lie with party dynamics and game feel.

I think Casters & Healers having less defense and HP is a good thing. The benefits they bring to the party, especially Healers, and helping them survive from Raid wides or their own mistakes is some of the last remaining bit of teamplay we have in this game in non-savage/ulti content. If a melee gets a vuln stack and eats an AoE you know they're gonna be fine. But if a Healer eats a vuln and you see the raid wide coming you know that it's beneficial for all to help them. Makes you feel like a team player and you know the other player feels thankful. It creates small community moments there.