I’m only guessing, but given Pepsi and Coke’s market strength by 1975, it's a fair surmise that Dr Pepper might have become something of a counter-cultural alternative for Flower Power 1970s teens and 20-somethings, despite its invention in 1885.
What happened in 1980 to cause such a drop off until 1984 ? Perhaps this 1980 TV ad starring David Naughton ('Must drink Dr Pepper...Must join Navy', as one wag says in the comments box) meant it lost critical cred among lonesome writers shivering away in Greenwich Village garrets. And what sparked the drink’s literary revival thereafter, well, until roughly 2000?
Quote: Well, I couldn’t find one for this article, but one reviewer of Elizabeth Berg’s 2006 novel A Year of Pleasures counsels other readers: “Read with a Dr Pepper – a drink that gets mentioned so many times you start to crave it – and Indian food leftovers.”