Sadly, I suspect that it is orange (and always will be) in the event, however unlikely, that something happens. Presumably,that will help insulate the responsible parties from some of the criticism that will follow. example: "We weren't asleep at the wheel! We suspected that something may happen - We were at threat level orange after all!"

If we drop the threat level to yellow and somebody tries to bring a gallon of water through security, the TSA will look incompetent.

The really dumb thing is that we call this "a heightened state of security". Heightened from what? If yellow or green effectively do not exist, aren't we just at the 'baseline' state of security?