There has never been any death directly caused by marijuana, while the deaths caused by cigarettes is significantly higher.
Because pot is arguably less addictive, and a lot less people smoke pot. Also, yes, people can get cancer from the carcinogenic chemicals used in pot... but these cases will probably be attributed to 'smoking' because it isn't directly linked to the 'high' part of smoking marijuana.
Also, that something does not kill you does not make it harmless.