I think there is a little confusion - they aren't developing a sitting segway as far as I know or guest use in the parks...Disney is working on (really, getting someone else to supply) a new standing vehicle that has four wheels. IIRC, one of the issues brought up with the segways is the "failure conditions".
A segway, in operation, balances on its two wheels via a computer that constantly adjusts the balance via a series of gyroscopes. It is not a "naturally stable" condition. Before you turn it off, there is a "kickstand" that needs to be lowered, giving it a third contact point and more stability.
It actually isn't hard to over-lean on a segway and topple, especially if you are not familiar with it.
A four-wheeled unit is inherently more stable, and is naturally in a "failsafe" condition - if the unit power fails suddenly, it will stop...but it won't fall over. I don't believe it is the same for a segway (there are fault detection systems, but I believe the parking stand is still manually deployed).
A wider wheelbase also adds even more stability during use.
Edit: I forgot...Segway did do development on a 4-wheeled version of their own, meant for more off-road type applications. But it was also still supposed to be computer balanced, so I'm not sure if it would apply here...also not sure it ever got very far.