In fairness to the original concept of Stansted as a new London airport, Luton did not have the same space to expand as Stansted and the vision of a new build multi runway airport does have attractions. In truth though the only way to pay for it was to allow BAA to charge more at their proftable airports through LHR and LGW and channel the money into STN redevelopment. In doing so they also took their eye off the ball at both airports to the point where preT5, Heathrow was falling apart at the seams due to investment in shopping malls and nothing else.
With an eye on the future and T5 going up, BAA fell out of love with STN given their only major customer was FR who had them by the balls. Once Air UK went the way of the dodo, STN became loco central and the market it had been built to serve never materialised. The end product is a good airport in the wrong place with more facilities than it's main customer wants to pay for.
It was the last shot at a strategic aviation policy.
The market never wanted STN, the government and the BAA tried to buck the market and paid the price.