Tobacco has been used regularly for way longer and in much larger quantities than cannabis. There's been much more attention on tobacco as it is something that through the past few centuries was widely spread across society.

Widespread usage of cannabis is something moderately recent.

Cannabis is also consumed in smaller quantities: an average user may smoke one or two cigarettes a day, while a tobacco smoker easily smokes 10 to 20. Moreover, tobacco is legal while cannabis has been illegal in most places until very recently when it started being allowed in a few countries. This leads to much more data regarding tobacco available than for cannabis.