Why is the Floating Maul a two-handed weapon?

Weight and mass are two different things, would be my answer.

Wielding it with one hand would cause clumsy... would be mine

It the way the weapon is constructed is my guess, if you take the form of a big 2 handed war-hammer and remove the weight, you will (first off make the warhammer useless), second off you would still lose a lot of the momentum in the attack by wielding it in 1 hand, the core idea is, the weapon is made for you to swing it with 2 arms and apply as much force as possible (and as much of the war-hammers weight).

Sincerely
~ Kongdej

There's no reason why. If the thing is magically weightless to the wielder, but clearly still otherwise has weight (or it would be useless as a blunt weapon, a whiffle bat would literally be the superior choice), then they're going to be waving a ridiculous amount of momentum and power around. You are made no stronger physically, because it's the enchantment moving the weight and not you, but the actual impact, the end result, is a mind boggling amplification of your own strength.

Pick up a piece of wire and swing it as fast as you can back and forth. Now imagine if you could do that with a maul. The issue of size and leverage is kinda irrelevant considering you can walk at an enemy waving this weapon at them like a hyperactive blind man with a cane. They can't stop the damn thing. Unless they're a bone ogre. Then they laugh, take it off you, and smash your face in with an entire tree.

I am sorry, the enchantment only starts working when you touch the hammer with both hands... its a safety feature.

~ Kongdej

If you start thinking too much about magic, none of it will make sense. While it may be feasible to wield this weapon in one hand as well, it's probably two-handed for a good reason. Maybe the impact area vs the force it is wielded with would be too low otherwise, making it inefficient.

Well if we're talking magic, it could well have no weight but still the proper amount of inertia - meaning it still takes two hands worth of force to swing the thing, and it still hits like a two handed maul, it just doesn't take any extra effort to lift it. Imagine the magic as a constant upward force that cancels out gravity, it wouldn't propel the thing any faster when you swing it, you'd still need two hands.

Edit: it's a silly example, but this can occur in real life. As a thought experiment, picture a war hammer suspended from a blimp, hanging in midair at just the right height. It takes no effort to lift it, but to move the thing it takes enough force to move both hammer and the full weight of the blimp with it (blimps do have weight, just a little less weight (well.. density to be precise) than the surrounding air). Replace the upward force the blimp provides with 'magic', and voila, you have a 'weightless' hammer that takes a great deal of strength to actually swing.

Should be one-handed, good catch.  No reason to over analyse it.