Because it isn't needed?
Correct, the airport isn't needed. 2 half loaded Dorniers a day doesn't justify £4,500,000pa of public subsidy. Locals voted with their feet (cars) a long time ago to go over the bridge to Edinburgh for speed, frequency, price, proper aircraft and convenience, to name but 5 good reasons.
Compare fares alone: next Monday, first/last flight lowest day return fare:
BA is £105 to Gatwick, £235 to Heathrow, £302 to City, Easy is £46 to Lutton, £70 to Gatwick, FlyBe is £145 to Heathrow, £186 to City and £271 to Stanstead.
And each Stanstead seat is basically also supported by about £400 of public subsidy to make it possible.