Because to get the profit, they would have to tender at prices the host airports would not be willing to pay??
I'm against NATS selling off any part of NSL - I have argued long and hard that to be a 'world leader in ATM' we need the experience of doing the job at the coal face, but until certain contracted airports start making NSL money, why would a supposedly shrewd businessman keep them in his companies profolio?
Potential to make profit is all very well, what with cost pass through etc, but until you realise that potential, it counts for naught.