One story to explain it: Space filled a psychological niche for people, which something else now fills. A logical thing for that to be would be the internet and computers. For some of us who are hard core futurists, we also have far-future aspirations like life extension and uploading to concern ourselves with. It's not that we've given up on space, but it is less captivating because it competes with other interesting things now.

This.

Once upon a time the future meant space ships. "Go farther, faster." The future was the flying car, flying farther. The Jetsons. That was before DNA, before nanotech, before life extension, before uploading, before the internet. Before the more interesting stuff we can do right here.

I think space travel has a special escapist appeal to it. There are massive intractable problems with human society on earth at the moment which lack easy solutions (poverty, aids, overpopulation, climate change, social order). Space allowed the last generation to write stories where those didn't have to be dealt with because the protagonists could leave earth behind, but now we've realised we're stuck here and have to fix things.