Why it is not possible to enable airplane mode automatically after sleep tracking starts?

Unfortunately, due to security restrictions introduced by the Android team in 4.2, there is no way to automatically enable airplane mode from an app. You always have to go into settings or long press the power button. If you have a rooted phone you might consider using https://play.google.com/store/apps/details?id=lv.id.dm.airplanemh we have support for this in Sleep. A similar hack exists for 4.3.

If you don’t agree with the Android team’s design decision, you can upvote issue 40497 here http://code.google.com/p/android/issues/detail?id=40497.