Why do Wizards use 1-hand weapons if 2-hand weapons hit harder?

Because if you use 1 hand weapon you can benefit from an offhand item that bring you a lot of stats as well as damage that you wouldnt be able to find on a weapon. You can find

• Crit chance
• Elemental damage
• Vitality
• Some good damage that compensate for not using a 2handed weapon
• Arcane power
• Bonus to a skill
• etc

Since 2.0 patch, elemental damage and +x% skill damage has became the bread and butter of every classes. Imagine you "lose" 10% damage by not using a 2 handed weapon but you gain 15% to blizzard on your off-hand, in the end you benefit more from it since you gain more damage.

Currently I don't see why any class would use a 2 handed weapon (Except the Crusader for obvious reason)

What WizLiz said is true.

However I would still probably choose a 2H weapon if I want to make say a Disintegrate or a Ray of Frost build, because the slow attack speed will make my arcane power tick slower and I can hold the ray for much much longer (about 30 secs with skills).

Also the damage is really nice and not to be ignored even without the extra stats from the offhand.

The Disnintegrate/Blizzard build (pre RoS), which was the next most popular one, next to the CM wizard, was played with a Skorn about 90% of the time.