I've been travelling through Luton since 2003 - it's a dump and always has been. Certainly, the opening of the new terminal area in 2005 was a big improvement, but it's not a particularly enjoyable place to hang out.
However, to turn Luton airport into a pleasant environment is going to cost a ton of money. Stansted is nice only because BAA spent a fortune on the general infrastructure over the years.
Neither LBC nor Abertis really care if it gets the odd bad press article - they care only that the revenues keep on coming - ie Joe Bloggs buys a seat on a flight and spend his money in the airport shops
Runway and overnight ramp capacity around London is in short supply and very few airlines *really* want to take on MOL at Stansted - so Abertis may figure that it's not really necessary to spend a vast amount of cash on a nicer environment at Luton - the passengers and airlines will keep on coming anyway. Abertis' shareholders will be more keen on any capital investment being spent on expanding airport capacity instead so as to increase passenger numbers.
The people who see the grotty bits of Luton airport are very much flying on LCC or charter airlines and their passengers' decision to fly takes price very much into account. If you want large scale building works at Luton, the money has to come from somewhere - namely increased airport charges which translates into higher cost of flights. Joe Bloggs may not like Luton much and may make a big fuss to the press, but does he *really* want to pay a higher ticket price in the future for the nicer environment ?