Because it might be unsafe. See, the human brain undergoes important development until the age of 25 and research has shown that cannabis significantly affects the brain of children and adolescents. Structural changes have been found in the brains of young cannabis users, especially in the prefrontal cortex, the last brain region to undergo maturation during adolescence. I'm for legalizing medical as well as recreational use of marijuana for adults but I think more research has to be done on how certain cannabinoids affect the brains of children. Especially because cannabis might be the only known substance that helps for example with Davet syndrome - it can save their lives.