Because physics is full of magic! Sadly I don't have enough physics background to google this phenomenon in a way that provides good results results, so I can't

Because physics is full of magic! Sadly I don't have enough physics background to google this phenomenon in a way that provides good results results, so I can't find other sources, but the way I understand it is there is a small band of radiative frequency that the atmosphere (at least on a non cloudy day) is almost completely incapable of absorbing, and so the energy is essentially easy to radiate into space. If I had to guess, it's probably related to the chemical makeup of our atmosphere, and a different gas and particulate makeup would create a different frequency band. Mirrors don't work in the way you're suggesting because they don't provide enough of a shift in frequency in the energy they rebound (though they would act as an effective shade). For active cooling strategies, what you want is a material that can take the infrared frequencies from a heatpump, and shift them to the specific frequencies that make it hard for the atmostphere to absorb (similar to the use of phosphors in lighting for the visible spectrum). Edit: I clearly didn't read this particular article closely enough, and the process they're measuring is actually a passive cooling system, which shows the material, or at least it's shade to be cooler than ambient temps. You could still use this material in conjunction with simple radiators to get better performance from a heat pump though.