Because while they want you to think that 4000-7000 people who use segways as a mobility device are a significant number, the truth of the matter is that they represent such a minor number of the general public that specificly accomodating them is well beyond "reasonable."
US Population: 301,139,947
People who visit the MK in a year: 16,640,000
Number of folks on segways in the population? 0.00002%
Assuming an even mix.. that's only 332 people in an entire year who would use them showing up at the MK.
Even if every single person who uses a segway (using their high estimate) goes to WDW in a year, they would represent only 0.0004% of the MK's population.
Now we all know not every single person is going to go, but that all works out to the chances of a person on a segway as a mobility device being in MK on any given day is slim to none. The numbers dont even work out to 1 per day out of MK's ~45k guests.
Wheelchair users are about 1.5% of the population or about 4,517,099 people. In terms of the MK, that's 249,600 people per year. That's 683 per day at the MK.
Less then 1, or 683. Hmm.. which should we accomodate? Which is reasonable to accomodate?