Oyster cards are quite primitive really and only have four bits (1s or 0s) that can be used to define the zones for journey history/charging purposes. Hence a maximum of 15 zones can exist in the Oyster system (2 to the power of 4, less one for "no zone" presumably).
Gatwick simply had the good fortune to come first in terms of the Oyster rollout, and so the highest zones are used for travel to Gatwick. The only way of expanding Oyster to be valid at Stansted would be "align" the fares (i.e. increase them to the extortionate Stansted rate), which I'm sure would be popular...
Gatwick simply had the good fortune to come first in terms of the Oyster rollout, and so the highest zones are used for travel to Gatwick. The only way of expanding Oyster to be valid at Stansted would be "align" the fares (i.e. increase them to the extortionate Stansted rate), which I'm sure would be popular...