If anyone had enough advance knowledge of the cargo and the flight it was carried on to plan to hijack the plane, they would have saved themselves a hell of a lot of money, risk and trouble by hijacking the truck that took it to or from the airport.
Anyone who was capable of organising the heist of a 777 in order to run off with its cargo would probably be able to make darned sure they had the right contacts in place to ensure it went on the right aircraft at the right time.
The problem with hijacking the truck on the way to the airport is that you would then still have to get it out of the country. That problem is eliminated if it's already 'flown', as it were.