Without trying to understand the mind of the designer, and ignoring aesthetic considerations (of which I am certain are part of this decision), I want to look at just weapon design / wielding rules.
Staves are required to be wielded with hands near the middle. This is why they make terrible fighting weapons. They are basically just unwieldy short swords. When blocking, though, this rules means that the wielder has pretty good control of the weapon, even when wielding it with only one hand. At worst, the pivot point is only a short sword length from the point of impact of the weapon being blocked (and is often quite a bit closer).
For any other two-handed weapon, that all changes. It is legal to wield other two-handed weapons with hands nearly at the butt end of the weapon. This is great for getting reach while attacking and almost makes up for the slow speed of the weapon (does make up for it in the hands of some very skilled individuals). On defense, though, this would potentially create a very unsafe situation if it could be wielded in only one hand. With the pivot point potentially almost 6' from the point of impact, a single hand simply doesn't provide enough control to keep the weapon from swinging wildly. Heck, from my observation of combat, sometimes two hands don't provide enough control (but thankfully not often).
The elephant in the corner here is the spear rules. Spears are pretty much just slightly shorter, less padded, off-kilter staves when used for defense. And if wielded in the wrong way, they would present almost identical problems to two-handed weapons on defense. However, without any rules forcing it, the culture of spear use on defense has resulted in a style where almost every person wields a spear defensively with one hand about 1/3rd of the length of the spear from the butt end and usually hanging down (which is quite stable). And while this same style would probably help with stability for a two-handed weapon, the difference in weight (two-handed weapons are longer and bulkier) is enough that this would probably be a lot less safe if wielded this way by the average player.
I don't know that safety was a consideration, but I think that there is a safety issue that can't be meaningfully ignored.
-MS