Except Machinist, Bard, Ninja, and Monk don't really have options for heavy armor either. I could go on and cite the idea that heavy armor wouldn't be conducive to the range of motion or mechanics related to the class and that plausibly Ina sense they may lack the particular Constitution to make heavy armor practical in combat as they require greater mobility but it's not about being sensible, it's about you wanting to look a certain way. The idea of allowing cross class glamour also has PvP implications due to visual indicators where small text is not plainly readable, IE "SMN" wearing the Paladin Valor set or even certain Tanks swapping specific sets but that's more or less a different argument entirely. That aside, Lore reasons probably.