Okay, some thoughts on possible reasons for this trend:
* Cyberpunk and the notion of The Singularity shifted the SF discourse towards on-Earth or in-System futures to the point where postulating FTL travel uses up most of a would be SF writer's credibility points. Adding living, relatable aliens on top of that started to feel old-fashioned and uncool, like trying to sell people on ray guns in a Hard SF story. Grim Meathook futures don't get to have happy first contact stories!
* Media mainstreaming and backlash. Whether it's overexposure to Star Trek (c.f. timprov
), too many flying saucer aliens in The X-Files, or something else, aliens may have hit a cultural saturation point, leaving writers tired and/or bored of them. Alternately, people might imagine that TV could/is handling them better than they could, or book buyers at chains could've decreed that aliens were over. Either way, the result is less Planets & Aliens stories.
* Growing discomfort/disinterest in "Frontier" narratives. A lot of the ways colonizing alien planets get used as a metaphor for the American West are troubling, and further, the Western and visions of frontiers have taken a dive in terms of cultural relevance. This loops back to the first point, in that as manned space missions taper off, futures where human can reach other planets and meet aliens begin to seem less and less credible.
I'm sure there are other possible explanations, but I'm getting kind of recursive here. Thoughts?