It's not just that the U.S. is a big boat, it's that the federal government is necessarily slow to respond to local changes. At the national level, the consensus in favor of legalizing marijuana shifted to a slight majority just last year: http://www.people-press.org/2013/04/04/majority-now-supports.... Indeed, if we're talking about the voting public, rather than just the general population, the national consensus is likely still in favor of keeping it illegal, since actual voters skew older and more conservative than the population as a whole.

Furthermore, the drug policy boat in particular is very big. A lot of this policy, and the institutions implementing it, was developed in the 1980's and 1990's. In 1991, the consensus in favor of making marijuana illegal was an overwhelming 78-17. Being anti-drug was a political winner, something both sides of the aisle could get behind. Almost everyone old enough to have substantial leadership within the drug enforcement organizations grew up in an intensely anti-drug culture. It will take quite some time for those folks to age out of the system.