This EXACT topic was done a few months ago. I will repost what I had posted there.
Pot is illegal cause of it's imposed "colourful" history.
Go back to the 1920s if you would; a time, way back when, when Mexican immigrants were seen as detriments to American society

. They habitually smoked "marijuana" (the derrisive Spanish word).
There was a time when hemp fiber was an incredibly common fibre. The potential of having hemp in our day to day life (from fuel, to fabric, to virtually any kind of material you could imagine) is astounding but is and has always been blocked.
Back in the 20s Dupont had just developed polyester; the "fabric of the future". Hemp was seen as a dangerous competitor and Dupont took VERY offensive action against the (fairly extensive) hemp and hemp farming industry. They had a public smear campaign.
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Do you know William Randolf Hearst? Now there is a fine upstanding citizen...nooot!
He single handedly managed to put hemp into the full nelson. See, ol' willie hearst, a huge newspaper mogul, owned a rather extensive patch of land. The trees on his land were very valuable as material for paper with (you guessed it) hemp being it's competitor. Hearst viewed hemp as being the single greatest hurdle between him and his (even greater) fortunes. So, in his newspaper, he ALSO began a public smear campaign and ALSO aligned important politicians and FBI heads to his cause.
If one were to read the transcripts of the presentation of the bill, one would laugh...and cry.
Since that time, hemp's influence in our daily lives has been fairly limited.
>I apologize as I see jayem has already given some very good info.>...but if I may continue....
There have, since the 30s been some oustoundingly ridiculous tests to prove the dangers of MJ smoke.
One instance, in fact, where monkey were gas masked for 20 minutes with high grade colombian MJ smoke.
As I see it, there is no LOGICAL, intillgent, or meaningful reason why marijuana is illegal.