Probably because at least 91 per cent of U.K. teenage illicit drug use is cannabis. In the U.S., cannabis accounted for 82% of past-month illicit drug use by 12-to-17-year-olds, but only 72% of substance use disorders.
“Cannabis is responsible for 91 per cent of cases where teenagers end up being treated for drug addiction”
I know in the USA teens are often sent to rehab in lieu of jail for cannabis first offenses. Do they do that in the UK? That would explain the statistic.