I agree...why not?! They don't distinguish in FL do they for special deals at WDW!!
Florida has 18M residents. SoCal alone has over 23M residents.
It would be a disaster to extend SoCal deals past just SoCal.
In the most laymen terms I can think of, here is why it is SoCal-only for economic reasons (and beyond this, there are marketing reasons):
The above is a simple supply and demand graph. The red line is supply, the blue line is demand and the dot is the optimal price to sell the good. That is how Disneyland will set their prices.
Now, when you take into consideration promotion pricing, the demand goes up, profit goes down, but the maximum supply remains constant.
Assume that everybody is charged the same price. In that case, profit equals x.
So when Disney reduces prices for a certain amount of people, it needs to do so cautiously in order to keep an optimal price. The swing to the left on the blue line made by SoCal residents needs to be made up for by a swing to the right on the blue line for non-SoCal residents. Otherwise, optimal price is not reached, and that's simply not good. So assume profit from SoCal is now "y" and profit from non-SoCal is now "z." Disney still wants profit x, and they'll still make it. The difference between y and z will equal x.
Baja California (which qualifies for discount tickets but not the payment plan) is added in because adding it is of negligible difference to the supply curve as a whole (same reason why select Southern Georgia counties qualify for Florida Resident at WDW). Adding NorCal is not.
Disney knows what they are doing. They would destroy themselves if they offered this to everybody in California, because demand would never be able to keep up with supply.
Furthermore, they need to offer a marketing incentive to get locals, who can come often, to the park - the incentive is discount tickets that have a delayed expiration date. Out-of-town guests benefit from hotel discounts this summer.
