Just as the title states and the question is more for Anet to answer because everyone’s opinion will be different and would really like a legitimate reason from them.
As a note: some might say “well it gives them room to add more weapons to other expansions.” Well to be honest I don’t think that’s a good idea because after EoD, they can easily just introduce new elite specs without having a weapon to be tied to it. Take Holosmith for example, they can just add “weapons” onto skills for future elite specs for other professions.
Additionally, an elite spec that’s capable of dual wielding diversifies weapon composition. So if I’m a Mesmer and only got a Pistol for off hand, why not be able to have it in main hand? (Dual wield pistols would be a dream too!)
But regardless, with that said, below are just a couple of professions that I think would’ve made sense to give them dual wielded weapons to their core/elite.
Dual wield (Sword): for Guardian, Ranger, Thief and Elementalist.
Dual wield (Axe): Revenant, Mesmer, Necromancer
Dual wield (Pistol): Mesmer
Dual wield (Mace): Guardian, Revenant