WHEN Disney struck a deal just before Christmas to buy much of 21st Century Fox for $66bn, it was a career-defining moment for the two firms’ bosses, Bob Iger and Rupert Murdoch. A third media mogul, Brian Roberts of Comcast, was left out in the cold. Having tried and failed last autumn to get Mr Murdoch to take a higher offer, Mr Roberts may now be preparing a still richer bid to upend the deal. It is not hard to understand his motivation.