That’s because those rules are in the sporting regulations. Appendix 7

The following restrictions apply during RWTT: a. Only wind tunnels that use air at atmospheric pressure as the test fluid are permitted. Other than rotations of the RATG and model or ground plane about the yaw axis, designs which attempt to create curved flow conditions relative to the RATG are not permitted. For closed section wind tunnels adaption of vertical walls and the ceiling to improve air flow uniformity is permitted. Particle image velocimetry systems where the wind tunnel air transports a flow visualisation medium are permitted. b. No RWTT may be carried out using a scale model and RATG which is greater than 60% of full size neither may it be carried out at a wind tunnel air speed exceeding 50m/s measured relative to the scale model and RATG. Furthermore, during restricted wind tunnel testing the magnitude of the rate of change of the wind tunnel air speed measured relative to the scale model and RATG must be less than 4.5m/s². The rate of change of the wind tunnel air speed will be defined as the derivative of wind tunnel air speed and smoothed using a moving average filter, centred on each sample, of period 0.5 seconds during each wind tunnel air speed ramp up and ramp down phase. These phases are defined as the periods when the wind tunnel air speed is varying between 15m/s and 95% of the maximum wind tunnel air speed during a run.

There’s quite a bit about it in there. You should read it some times. https://www.fia.com/regulation/category/110