In the 1960s, the comic book industry saw the beginning of a new phenomenon. The average duration of a comic book reading "generation" had traditionally been about 5 years, before the predominently prepubescent readers would "outgrow" comics and move on to other interests.
But now, thanks to new approaches to characters and storytelling, readers were not "ageing out" as they always had before. And, as the median age of comic book fans increased, the interest in more complex and mature storylines increased as well.
But a little problem known as The Comics Code Authority stood in the way. In response to US Senate hearings in the 1950s on the alleged link between comic books and juvenile delinquency, the comic book industry pre-emptively imposed a strict set of content guidelines on itself before the government could do so. So, while The Code made comic books "safe" for impressionable youth, it also made for rather limited content to appeal to older readers.
In 1971, Marvel Comics finally became aware of what magazine publisher Jim Warren had already figured out years earlier, that black-and white comics published in magazine format were not subject to The Code, because by definition they were not classified as comic books.
And so, with the publication of Savage Tales #1, a new chapter of Marvel Comics began.