The Telegraph lists the 50 best cult books.
Whenever there's a list like this, I like to construct a histogram from the list. For series, I'm arbitrarily using the date of the first book in the series.
Unsurprisingly, like every other such list it approximates a bell curve, this one peaking in the 60s. (That early bump is an artifact of condensing the 18th and 19th centuries.) While the argument can be made that that era was more conducive to these sorts of books catching on, I think what we're seeing here is a skew from the compilers of the list and society in general. This peak corresponds to the postadolescence of the Baby Boomers. Cult books from previous generations are largely forgotten; cult books from later generations haven't attained the same level of sustained notoriety.
Commenters have pointed out some of the more egregious omissions, and I think it's telling that (at least among the ones I agree with, which is admittedly biasing things), they're either more modern works that haven't gotten credit (Fight Club, Trainspotting), or genre works (Stranger in a Strange Land, Lord of the Rings).