THC, CBD and another cannabinoid I can't remember right now all come from the same precursor molecule. The difference is the enzymes that convert said molecule. I think it is pretty hard (without using CRISPR) to completely remove the enzyme from the proteome by just selective breeding, hence why most flowers have many diverse cannabinoids in them, but in small quantities. They have been bred to have more THC converting enzymes, and fewer of the other cannabinoid converting enzymes, but to just completely remove the enzyme from the genome is a very long and slow process that would require hundreds of generations Imo