And much harder to pack and ship. You could probably fit 1000 cardboard beds in the same volume of a cargo hold as 100 metal cots. Also, there are many local shops who already have the tools to mass-produce cardboard boxes with arbitrary shapes; the same cannot be said of cots.
And as the article explicitly states, these are intended for the first responders, not for long-term infirmary. They don't need to last more than a few days.