The answer is very simple, profit
But it's not pug. The airport has a history of damaging acquisitions that have ultimately distracted management.
From memory, the dividends received by the councils over the years have been very low from an asset of this worth. The focus should be on increasing dividends not spending time raising external funding from equity and bank funders for an acquisition that has been competitively priced (i.e. it's not an offmarket, low price deal).