Criminals have been making ever stronger strains of skunk that have less of the chemicals that seem to help prevent psychological issues. By putting cannabis use into a legal framework, authorities could ensure safe limits, much as they do with alcohol and tobacco.
I'll say it again, current methods don't work. You say it yourself, strong cannabis is readily available in the UK and is used in high numbers, even though it's already illegal. While drug control and distribution is in criminal hands, there is no control over the safety of what people use.