Pollywogs!

Police in the United States made a record number of arrests in 2005 according to the FBI's annual Uniform Crime Report. The 786,545 people arrested for marijuana violations made up 42.6 percent of all drug arrests, and more arrests the total number for all violent crimes combined, including murder, manslaughter, and rape, robbery and assault.

From : wired.com 

One third of US citizens have tried Marijuana.  Prohibition has proven itself a useless excersice.  The only things I can think of:

1)  Government wants to 'look tough', and repealing prohibition would cost SOMEONE votes.

2)  Drug Lords employ lobbyists just like anyone else.  Repealing prohibition is bad for their business as it would really cut profits (if prices were then raised with taxes to fund rehabilitation and education, as it should be, which would also reduce their business, the profits would then go towards something useful beyond beer, bitches, massive mansions, and substantial financial consideration to our very much in-need elected representatives…).

3)  Marijuana makes black men rape white women…probably has the same effect on macaca's and other ethnic types.

4)  Marijuana will make your children have sex with said ethnic people…

5)  …and probably crash a train into a bus full of nuns and orphans…

6)  …or spend the next decade playing Playstation in your basement. 

7)  Marijuana contributes to the American obesity crisis. 

8)  Et cetera, ad nauseum.