Why can cannabis be legal at a state but not a national level?

Cannabis can be legal at a state level but not at a national level in the United States because the U.S. Constitution grants states the power to make their own laws and regulations. Under the 10th Amendment, states have the right to regulate "police powers," which include issues such as public health and safety.

As a result, states have the ability to legalize or decriminalize marijuana possession and use, while it remains illegal under federal law. This creates a conflict between state and federal law, and creates a legal gray area.

The federal government has not yet repealed the laws that prohibit marijuana use, possession, and distribution. The Controlled Substances Act (CSA) of 1970 lists marijuana as a Schedule I drug, which means it is considered to have a high potential for abuse and no currently accepted medical use. This makes it illegal to grow, possess, or distribute marijuana under federal law.

However, in 2013, the Obama Administration issued the Cole Memorandum, which instructed federal prosecutors not to prioritize the enforcement of federal marijuana laws in states that had legalized marijuana. This policy has been rescinded by the Trump Administration, but under the Biden Administration, it's not clear yet the stance towards the state legalized cannabis.

This means that while states can legalize marijuana, it remains illegal under federal law and this creates legal uncertainty for growers, sellers, and users, and also limits access to banking and other financial services.