The first issue of Playboy hit the stands in December of 1953. The magazine espoused a philosophy that was pretty radical for that time, namely that sex was not just not bad but actually good and fun and something everyone should be doing. It was "sex-positive" in a time when literally nothing else in the culture was.
This boldness served them well from the '50s to the mid-'60s, as the rest of the culture slowly started to come around to the same point of view. The problem is that by the late '60s the culture had reached the same point that Playboy had, and it didn't stop there -- sexual liberation kept galloping on, reaching points that were far beyond anything Playboy had ever advocated for. By the early '70s, for instance, American culture was so saturated with sex that pornography was just another part of the cultural landscape (see https://en.wikipedia.org/wiki/Golden_Age_of_Porn). Playboy had agitated against the censorship laws that had kept such films out of general circulation; but when those laws disappeared, it had nothing to say about the results.
Playboy, in other words, eventually got lapped by the changes it helped to create. The culture galloped along, but the magazine didn't do anything to keep up, so its relevance slipped and eventually tumbled. It was still selling a Mad Men view of sexuality in a world where divorce and cohabitation and premarital sex had all become part of everyday life. In the 1950s, Playboy was daring and edgy; by the 1970s, it was positively quaint. And while porn can be a lot of things, one thing it can't be is quaint. Nobody ever got turned on by something that was 100% safe, 100% familiar.
So Playboy started dying way back then, when it gave up its claim on cultural relevance, and the story of the decades since has just been the slow playing out of the inevitable.