The problem is that cannabis was never shown to be more harmful than tobacco scientifically in the first place, but the public and politicians were led to believe that it was. By the 1950s equivalents of "fake news".
You can see a similar "manufacture of risk" process going on with vaccines at the moment, which is very dangerous. It's also routine for the UK press to engage in ridiculously exaggerated anti-drugs campaigning, as it fits their sensational moralistic worldview.
Arguably more of a problem is the "we know this has risks but we can't feasibly ban it" category: alcohol.