I think Long Haul points to the right main criteria: long haul flights (ie lots of fuel), common types, and early morning (to minimize the risk of flight delays). That the flight loads were light may have helped them in the sense of making it easier to control passengers, but I suspect was most likely happenstance. I'm not sure that they needed some infiltrator on the ground (which doesn't mean they didn't have any). If all they brought one was knives and threats ("we have a bomb!") they could easily pass those through the detectors.