The difference is that Stansted doesn't need a fortune spent on it. Gatwick was aging and a transformation into a more modern airport was needed, especially for the non LCC's like BA. A brand new check-in hall in the North Terminal for BA, a new security area in the South plus many more improvements have all been completed since GIP's purchase.
Stansted doesn't need that transformation. It's already large and modern enough but it currently overcharges, as the CAA has found, and since little needs to be spent on improving the infrastructre, MAG may be able to offer more attractive financial deals to airlines much easier than you might think.
So for all those who worry that £1.5 billion for Stansted is too much, think again because Gatwick has costed GIP a lot more than that with all the infrastructure improvements they've had to make. Stansted costed £1.5 billion and that is what it is. MAG have already said they don't plan to expand or make major improvments at Stansted, and quite rightly so.
You're not thinking like a business
The thing is, it's not me doing the thinking. This is what MAG have already said they aim to do. They want to:
Attract airlines from rival airports Luton and Gatwick through financial deals and good customer service
Simon Calder's Q&A: Manchester airport buys Stansted - Business Analysis & Features - Business - The Independent
(Also mentions EasyJet as a target to base more planes and that Stansted is usually uncrowded and tranquil, so perhaps a go a trying to fight back from Southend and no need for heavy infrastructural investment like at Gatwick.)
Bring the success they've had with carriers at Manchester, namely Jet2, Monarch and EasyJet and try to attract them to open/expand at Stansted
Manchester Airports Group buys London Stansted Airport
And in the words of Charlie Cornish, Cheif Executive of MAG, the aim to fulfil Stansted's potential (note "potential" meaning it is already able to) as a "high quality alternative London access point for global air travellers."
Stansted sold to Manchester Airports Group | News | Travel Trade Gazette
By the way, WOW has not gone bust. They took over Iceland Express if that's what you're thinking.