Science Fiction as a genre came to life during a time when science was making enormous changes in people's everyday lives within a relatively short time. Railroads, electrification, long distance communication, mechanization, flight, etc. all either appeared or went "mass market" in a few generations.

This may well have been the first time when science and technology were brought to people's attention as something that actually had a direct effect on what they did. While before you could live in the same village for your entire life and not see much change, now everyone saw the impact of science when a railroad came through your town, or when you got an electric light.

Because of this, there developed a large market of people who were interested in both producing and consuming the "what if" scenarios and analysis provided by sci-fi. Today, the idea of change brought on by technology and science is not only an accepted idea, it's pretty much a given. Sci-fi hasn't run out of steam, it's just that the function that it provided has now diffused into every single other form of media.

It still reappears as a distinct form from time to time, accompanying major inventions such as the internet(Neuromancer, etc.) but more and more I think that it will just stop being a distinct genre and blend in with everything else.