When the Hippocratic oath was created, "deadly drugs" were limited to things like hemlock and arsenic that lead to horrific suffering before death.  Hippocratis could not have invision anything about modern medicine, but particularly not the existence of effective pain killers and their ability to cause death in strong doses.  While it is true that an abled bodied person has many options for suicide, the terminally ill may not have the option to buy a gun, prepare a noose or even make it to a lanai 4 floors or higher to ensure death.  Assistance with drugs may be the only option that they have.