Guardians are made to be Tanks in which Mobs just bash on while the other group members kill those Mobs. Having the highest AC and HP allows the Guardian to perform his job the best. Guardians arent meant to be damage dealers in groups. If Healers see a Guardian using a 2hander or dual wield, they will ask why, because that means extra work for them along with possible down time for the group.
Also, along with using a shield, Guardians have a few shield-only special attacks they can perform. They do not have any dual-wield-only special attacks or 2hander special attacks.
Now, when im soloing, I DO use a 2hander because it has better damage output and im usually not fighting the same caliber Mobs that I do in groups.