Others said it, but its because its just TOO easy for people to take advantage of the system if you come for vacation for a week from New York somewhere, put down a single down payment, use it for a week and then dont expect to pay the rest. I'm sure that happens for pepole living in the state too but the thinking is most people who live closer would do that much less. That said I'm sure in time more places will get the monthly payments. They will probably roll it out in closer states in the future but I doubt it will ever be nationwide, just from the states that provide many visitors ie, the closes ones.
And same for WDW, Florida residents has monthly payments now, but no where else yet!