The main reason cannabis is illegal is because the pharmaceutical industry spends millions of dollars lobbying congress to keep it illegal... they don't want the competition

The problem for them is public opinion has changed about cannabis.  It is no longer considered some drug that turns you into a demon and makes black people rape white women (have you seen "Reefer Madness"?)

Cannabis as a medicine has really changed how people view the plant.  Parents with epileptic children were moving to Colorado because they had access to medicine which stops their kid's seizures.  It is heartbreaking to see other families who cannot afford to move to Colorado not get the medicine their children need.

I have seen polls showing as high as 94% of Americans support medical marijuana, and 60-70% support recreational marijuana

Everyone sees the tide has shifted and cannabis will soon be legal.  Anyone with power (money) will want to invest in it... people love the stuff, and it will make boatloads of money, provide jobs, etc