Many things are illegal because they are perceived to be problematic. Unfortunately the perception may not be realistic. The perception may be based on prejudice. Alcohol for example causes problems, but prohibition only made it worse. The public has realized that the war on drugs has not eliminated their abuse and has had a lot of undesirable side effects even some politicians are beginning to notice. A lot of recent opinions express opposition to GMO, but the sources of their factoids are murky. A rumor can make it around the world before the truth gets its fact checking complete.
Although many governments have banned some GMO products, as near as I can find out there is not one peer accepted scientific study that finds anything wrong with GMO food. Sure you can argue to wait until it is proven safe, but with that logic we would all still be cowering in jungles afraid of fire.