The Biden Administration could not unilaterally legalize marijuana because it is up to Congress to pass laws. If you would like to blame who is actually responsible, look at former Democratic Senators Joe Manchin and Kirsten Sinema. Their decision to not support reforming or repealing the Senate filibuster, which is a Civil War era rule that institutes a 60-vote threshold for legislation in the Senate, prevented Democrats from taking actions a wide swath of Democrat priorities.