Even if that weren't the case, cannabis contains lots of psychoactive compounds which aren't fully understood, and even for the ones we do seem to understand, the brain and body are incredibly complex and chaotic systems and could possibly exhibit unexpected effects when exposed to them.
Also, 82% of the UK's population is white, so 97% (especially given that these are older people) isn't really that surprising. I'm baffled how pro-drug proponents consistently ignore correlative evidence that points to regular cannabis use being problematic.
“Discussions on the potential public health impact are hampered by gaps in evidence and variable quality of available data. Little or no mention of cannabis exists in CVD risk assessments or lifestyle advice guideline.”
Furthermore, they explain that much more research is required to come to any sort of conclusion on its effects on the general population.