The changing schedules are one factor. Two other big ones are
1. Irregular schedules, which can make it difficult to find time to exercise.
2. Airport food. Not terrible if you travel every once in a while, but when you've got xx minutes between flights, and you need xx+1 minutes to do what you have to do, it's a lot easier to get a burger and fries right next to your gate, than looking around the airport for something healthier.