I don’t think Cigarette smoking became unpopular without heavy regulation. In countries that don’t have such restrictions, smoking is still ubiquitous.
The data [1] don't really show this. In particular as recently as 2001-2003 35% of young adults (in the US) said they smoked, versus an all-time historical high of 45% across all age groups for some context. By 2019 that figure was down to 10% for young adults. The first bans on advertising smoking went into effect in 1971!
So what happened in the ~2000 era window of time? The internet and social media started exploding in popularity. In fact you'll find that the graph of the decline in teen pregnancy [2] looks roughly similar to the decline in young adult smoking, and probably for the exact same reason.
A much more obvious reason for the decline in smoking is the popularization of vapes. If you look at U.S. adults, you'll see that the decline in tobacco smoking is almost entirely made up for by the increase in vaping.
Technically, of course, vapes aren't cigarettes. But It's a bit misleading to say that smoking is unpopular, when it's just been replaced by a doppelganger. Especially since vapes are also deeply detrimental to health [1].
Indoor smoking bans were passed during that period. This was a time period of major pushes to outright ban smoking. That’s when I saw it actually drop off.
The things that did the most to drive down use were raising the price of cigarettes (which fund the healthcare costs) and banning smoking indoors, where it affected even non-smoking workers [1]. When we stop subsidizing harmful behavior, we get a lot less of it.
