Because People Can make their own decisions. It shouldn't, because apparently this is a free world, and some people like smoking and should be allowed to do what they like. Yeah, sure, you're gonna say it's unhealthy, but it's also addictive! It's those people's fault that they made that decision to start smoking and so we shouldn't interfere with that. Also, even if you DO ban smoking people will still sell cigarettes over the internet.
"I will kill you if you die on me Bob!"