With the benefit of 70 years of medical research this is an easy statement to make, but at the time this was far less clear - smoking was for a long time actively promoted as having health benefits [1][2]
It's easy from a modern perspective to look back and apply our accumulated wisdom, but it was not self-evident that smoking was necessarily bad. Otherwise millions would not have signed up for an early death.
[1]https://www.healthcare-administration-degree.net/wp-content/...
[2]https://i.pinimg.com/736x/40/50/4f/40504f0de983d78f3c0f0b22c...