I think science fiction in the U.S. declined suddenly because the standbys have passed away, by and large, of late, and there was a long stretch in the '80s and '90s where the "hard stuff" detoured into near future social dystopian fiction and transhumanist esoterica.
Me, I've always loved the sci-fi about advanced alien races and strange new worlds, but it seemed like that got very lost for a good long stretch.
Kind of in the same way Star Trek turned inward from being outwardly-focused over much the same stretch of time--instead of "strange new civilizations" we got long dystopian conspiracies and pablum-matic hegemonic characters (who ironically were battling an even more homogenous hegemony much of the time).
Right now, the book I'm most interested in reading is "Boundary" by Eric Flint... I like the crypto-scientific genre.
(Oh, one more thing: does anyone else find "military science fiction" to mean David Weber? I think Honor Harrington in particular might just belong to the other team--she seems a perfect representation of a *paladin* more than anything else...)