Up until the 60's, tobacco (and smoking in general) was widely perceived to be harmless and sometimes even beneficial. It is only recently that people started to realise that it is harmful.
I was wondering, why did it take so long? Tobacco has been smoked for centuries, if not millennia. It shouldn't take more than a few generations to realise that your friends who smoke have worse health than your friends who don't.
A related, but distinct, thread was asked here. Until you get a direct answer to your specific question, I believe the top post there from /u/aedes touches on parts of your question really well:
Smoking was nowhere near as prevalent before the 1900s as it would be by mid-century, when over half of US adults were regular smokers. The per capita number of cigarettes smoked per year in the US increased from less than 100 in 1900, to over 4000 by the 1960s. This was driven by a combination of the development of mass production technology for cigarettes, as well as marketing.
The implication of this is that many long term health effects of smoking were not well known prior to the early to mid 1900s - there were simply orders of magnitude fewer people smoking.
It is only recently that people started to realise that it is harmful.
Lord Baden Powell, of Scouting fame, wrote about the evils of smoking and included an assumption that his readers knew of the detrimental aspects of smoking (as would all sportsmen!) [though this might have been stylistic].
"Why is a boy who smokes necessarily a fool ? Well, I have given the reasons in a chapter in Scouting for Boys. One of the reasons is this : When a lad smokes before he is fully grown up, it is almost sure to make his heart feeble ; and the heart is the most important organ in a man's body. It pumps the blood all through him to form flesh, bone and muscle. If the heart does not do its work, the body cannot grow to be healthy and strong."
This is from Rovering To Success, p.75 (quoting himself in Scouting for Boys, 1908).
This article, "The failed history of tobacco harm reduction" gives further evidence from advertising that indicated harm from smoking was widely known/believed long before the 1960s.
Henry Ford spoke out against the dangers of smoking and wrote a book "The Case Against the Little White Slaver" (1916). His first volume includes a letter from TA Edison, and references notables of the day. Edison believed the principle harm to lie in the cigarette paper. The Lancet is quoted on dangerous aldehydes in the tobacco, etc..