Why doesn't the USA legalise cannabis at the federal level?

In 2012, Colorado and Washington made headlines when they became the first two US states to legalise cannabis for recreational use. Since then, 18 more states, as well as the District of Columbia, have followed suit by passing laws for distribution and recreational consumption.
Classified as a Schedule I drug by the Controlled Substances Act, just like methamphetamine or cocaine, cannabis remains illegal at the federal level. However, as more state legislators pass legislation to legalise recreational use, federal cannabis trafficking cases and charges have steadily declined year over year.
According to the US Sentencing Commission's 2021 Sourcebook, in 2012, the same year that Colorado and Washington first passed their legislation, federal prosecutors charged approximately 7,000 people with marijuana trafficking. Cannabis accounted for 27.6% of all drug trafficking charges that year, the most of any substance category.