Multiple US states have legalized the recreational use of Marihuana. Why is US federal law still anti-marijuana? A Democratic president, Barack Obama, was in office for 8 years and could have started the reform process. Congress could also move on the matter. The US has definitely liberalized culturally so the people’s representatives could advance this agenda. So what’s the holdup?
The problem is "the people's representatives". Many of them, especially those who have been in office for a long time, have nothing to gain by going against the status quo. They might also lose support from those who've bought into generations of anti-drug propaganda, and groups like law enforcement unions. So there really isn't enough legislative support, either at the state or national level.
It helps to look at how the recreational use of marijuana was legalized. IIRC it's currently legal in 10 states. Only one of them - Vermont - legalized through legislative action. The rest legalized through voter initiatives.
The medical advice is that cannabis is a biologically active substance (ie a drug) that in the short term can cause mild delirium (ie you get high), and sustained use causes mild dementia (short term memory loss). Smoking is also associated with a range of effects related to inhalation of particles.
It is the judgement of a majority of the elected representatives that these negative effects continue to be significant enough to maintain federal laws. And as the law already exists, and usage is relatively low (about 5-15% of adults use cannabis at least occasionally, compared with about 70-90% of adults using alcohol (wikipedia)), there would not be the disruption that could be caused by, for example, banning a substance that was comparably dangerous, but currently legal and much more widely used.