Before you say anything, "Yes, yes, another hit point thread... ARG!"
(Deep breath...)
Okay, so I posted about this in my other thread, but not wanting to derail that I decided to start fresh.
Why do different classes have different HD types?
Now, for the purposes of my question, I am making an assumption that you prescribe to the "abstract" HP camp where HP are a combination of several factors: physical endurance, mental endurance, skill, luck, favor, sixth-sense, etc. If you are in the "HP = meat only (or meat mostly)" camp then larger HD size makes sense for warriors and lower ones for weaker wizardy-types.
You could argue a fighter is "tougher" and can take a beating better, sure, but in the same light I can argue a rogue could have better luck or a wizard a better sixth-sense. Are those weighted less compared to physical endurance? Do you think a battler's skill is superior in combat so they get more HP? Well, wouldn't a caster be better at resisting the damage caused by other spells? HP don't differentiate between the source of the damage, so to say a barbarian gets more HP, even to resist the damage from spells, doesn't make much sense if those HP are earned during a career where the character mostly resisted weapon and natural attack damage.
Also, since front-liners tend to have better Constitution scores anyway because they want more HP, what impact would a flat universal d8 have? Would it hurt them that much, really?
FWIW, I don't really have an issue with HD, this is more about understanding a consistent and logical rationale for different HD sizes if you subscribe to the abstract HP concept.