DEA pointed to the statutory characteristics of schedule I drugs
Or in other words... "Because we say so"...
Psilocybin is an oddity. Its generally considered to be among the "harder" drugs, but afaik, it completely lacks the advanced addictiveness typical of the class. Pretty sure opiates present a much greater threat in that regard, and yet here we are :)
I can say anecdotally that psilocybin is a wildly effective means to abort a severe migraine episode... Something even the strongest of opiates cannot do.