One possible reason: Cardboard absorbs moist, which is neccesary for the virus to stay active. Non-porous surfaces don't. In addition, it was a lab experiment

One possible reason: Cardboard absorbs moist, which is neccesary for the virus to stay active. Non-porous surfaces don't. In addition, it was a lab experiment and from what I read, neither is it known how big the droplets were they tested with nor does anybody know how many virus particles are present in real droplets and how many might be transfered by smiring. Drying of droplets might happen faster or slower in practice, based on the environmental conditions. And before quarantining your next package for days, just remember: Washing your hands will get rid of the virus, it doesn't travel through the skin - you would have to rub it in eyes, nose or mouth.