Not sure how true it is, but I heard that Disney Dollars were being counterfeited, so they took them out of the stores to minimize the spread of them.(sorta have them all in a more "controlled invironment")
At any rate, printing paper money of any sort is definitely more costly than issuing plastic gift cards. So this is likely another reason that they are not as easily obtained.
I plan on getting some when I am there in 12 days for my kids, does anyone know if they pay the change in Disney Dollars?