Simple, Alchohol, and smoke does not damage your brain, nothing at all, they destroy your lungs and liver, which can kill you pretty fast, cannabis does not do that, BUT, is addictive and damage the brain, in small doses, it does not damage your lungs or liver, or stomach, or not much of your body; that is what got it to be illegal
Ever heard of alcohol intoxication aka drunk? The Hoff wasn't just rolling on the floor because of his liver. It's the effect of alcohol on the brain after it overwhelmed the liver. And there are nice long term effects:

As for smoke, ever heard of nicotine addiction?
And that's beside the release of tiny particles from burning plant-based material, which turn into tiny particles, small enough to pass through the lungs into the bloodstream to end up in one's brain where it affects one's intelligence. In that respect both cannabis and tobacco smoke are harmful to the brain, but it has been speculated the THC in cannabis counteracts some of the effects. Yet I think it would be wiser to remove the smoking part of cannabis ingestion.