Perhaps because we only remember the good stuff? Or only the good suff & famous authors get reprinted. I'm prompted to offer this hypothesis in response to Chad Orzel's commentary that there was a lot of bad space opera even during the "Golden Age" of science fiction. I recall that Zadie Smith once noted that 99.99% (or something to that effect) of Victorian fiction is forgotten and out of print. All that remains read are the "classics," so contemporary audiences have a biased perspective as to the median quality of Victorian-era writers. Of course the insight can be generalized to the arts overall; only the good gets reproduced or remembered. To be fair, some science fiction authors, such as George R. R Martin (who also writes other genres obviously), have claimed that the turn away from "hard" sf rooted in physics and engineering is reflective of a cultural shift. In the 1950s the "atomic age" was viewed positively, and the possibilities seemed unbounded. Today a lot of the fiction deeply influenced by science in constructing a story may take a more ambivalent, or even pessimistic, view of how science effects human happiness (e.g., dystopian futures where nanotech gets out of control).