The cynical answer is that sleep medications (and I mean the more highly scheduled varieties) are among the biggest pharmacological money spinners. Melatonin is cheap and unpatentable (although there was several attempts at melatonin mimetics, e.g. agomelatine). It's unfathomable that melatonin is available only on prescription (and has become increasingly hard to obtain). I think the toxicity is close to water... one would be more likely to gag on the number of pills necessary to O.D. and I'm not sure if there's even a single melatonin related death on record.