Or more precisely, why is science fiction losing mind share among the young(er)? I have a theory. Several actually, but I'll limit this post to one.
To wit: Not all SF takes place off Earth -- only most of it. And for many, to travel off-Earth must seem like yesterday's future.
Those of us of a certain age (i.e., gray- and non-haired) grew up with the excitement of the space race. The progress was dizzying: suborbital flights, orbital flights, orbital rendezvous ... . From Sputnik to One Small Step in twelve years. Inspiring!
But for anyone thirty five or younger, the manned space program has been endless circling of the Earth. For far too long, the program has been the international space station -- where nothing, besides construction and repairs, ever happens -- punctuated by shuttle disasters.
Not so inspiring.
If SF is mostly in space, and space is mostly (a) boringly repetitive and (b) your father's notion of the future -- why would you read it?
Judging by the numbers, you wouldn't. And that's a damn shame.
The purpose of space exploration isn't to inspire SF -- but one of the best uses of SF is to inspire space exploration. Because when we stop exploring -- and Earth itself is rather well explored -- we lose something of what humanity has always been about.