Unfortunately I don’t have an exact answer for you since there has been so little research done on MDMA since it was classified as a Schedule I drug by the DEA in 1986. The most common guess is in the 10–20mg/kg range, so a little over a gram for a 170lb person. According to wikipedia, several medical professionals argued for it to be classified as Schedule III but was overruled by the current head of the DEA, John Lawn. So, “the DEA did it” (and the War on Drugs) is also the answer to the second part of your question. Things seem to be swinging back the other way though, so who knows what the future holds for MDMA and other psychedelics.