the law
Cannabis remains federally illegal due to it's status as a schedule 1 substance.  Schedule 1 drugs are defined by the following characteristics:   they have no currently accepted medical use, they are highly addictive, and they are unsafe for medical use.  
"It doesn't have a high potential for abuse, and there are very legitimate medical applications. In fact, sometimes marijuana is the only thing that works." 
​- Dr. Sanjay Gupta, speaking about the issue of cannabis as a Schedule 1 drug