One factor is that, with the benefit of hindsight, it is possible to find what has survived over the decades as a classic worth reading. Old stories found in anthologies, and old novels still in print, are likely to be of higher quality than those that have disappeared. (Not always, of course. Like biological evolution, literary evolution is messy and sloppy and full of missteps and false paths.)
My personal quirks are also involved. I shy away from very long novels, which seem to be the trend these days. (I realize that's mostly a result of external economic factors, such as the price of paper.) There's something pleasant about holding a slender paperback from the old days (or the Doubleday hardcovers intended for libraries, which, if memory serves, were always 181 pages long.) I am also leery of endless series, and it chills my heart a bit to see an author's very first book touted as "Book One in the [whatever] Saga."
Some of it is simply the fact that my youthful reading tastes developed in the late 1960's and early 1970's. Just as I still listen to Classic Rock, I still read speculative fiction from that era (as well as the best of the older stuff and whatever grabs my attention as probably worthy of reading of the newer stuff.)
I don't think it's so much the content of the fiction or what the attitude of the author might be. I can greatly enjoy well-written Hard SF, well-written Soft SF, and well-written New Wave.
Please note that the number one factor for me is "well-written," which is very hard to define. Like pornography, I know it when I see it.