It's illegal in America because of two main reasons:
1 - Decades of misinformation and outright lies have been ingrained into the masses by the government and law enforcement.
2 - The powers that be stand to loose billions of dollars each year if marijuana is legalized. Those entities include the prison industrial complex, judicial system, law enforcement, pharmaceutical companies AND all drug cartels. All of which pay millions of dollars to lobby congress to keep cannabis illegal.
Things are changing slowly but full decriminalization will only happen when legislators stop filling their pockets with money from special interests groups and do what their constituents want them to do.