Why is cruising in level flight strictly prohibited within the Transition Layer between the Transition Altitude and the Transition Level?

1. Because engine thrust is unstable due to high humidity in that layer
2. Because aircraft above the layer use standard pressure (1013.25 hPa) while aircraft below use local QNH, making vertical separation uncertain and uncalibrated
3. Because radar surveillance is blocked by the marine atmospheric boundary layer
4. Because military fast jets hold exclusive rights to the transition layer