Well science fiction is science fiction because it is based on scientific concepts and theories that are either in a very rudimentar application status or not understood enough to make a pratical use of it, yet. People mentions fiction to justify it to not be based on reality at all, but fiction doenst mean not based in reality at all just means the story details are not fully to be found anywhere in real life, past or present. What people have been dfining in most posts so far is actually fantasy (not in the "official" art genre but in its broad genre), not fiction.
If you think about Julio Verne and his works in the middle of the 19th century he foretold things like the submarine, the spaceship and the telephone. But it based himself on knowledge and technologies available at his time, and depicted possible scenarios. Telegraphes existed already, and the telephone is like a second generation of land lines telecomunications, Verne imagined that step a few decades before it happened. There were submarines already at his time and even much earlier, almost 2 centuries earlier, but were basically prototypes. Verne imagined them reliable enough for a pratical use. And there were hot air baloons, which he explores in several novels, when he mentioned space flight he imagined flight taking a few more steps ahead, and wasnt that far either, not long after his dead the first attempts to launch rockets to space were made, and half a century later Russia was sending the first space shuttle and puting the first satelite in orbit, and further more 13 years US was making men land on the moon.
As for fantasy it uses concepts in nothing related to reality, mostly based in magic and religion. I don't say they can't coexist in the same fiction, they can and they do many times but are too totally diferent elements. Science fiction is rational, fantasy is irrational, in a sort of speaking. Fantasy allows a guy with a cloak and a scepter to cast frostbolts spontaneously from his hands, or to transform himself in a dragon, or to evoke tidal waves with some special words. No rules, no principles just imagination going totally wild.
What can make some science fiction look irrational its not so much related to concepts but I would say more to the extent some technology would be applied. For example, many times earth was imagined at the beggining of the 21st century covered with megalopolis, 1km high buildings, massive mag-lev like rail networks, huge widescreens, massive computer networks and everybody with their single or family size aircraft. That didnt happen and likely never will not because its not possible scientifically, it is, but because the amount of resources needed to do that are just not available. We probably would need to colonise other planets and undergo massive mining campaigns to transform earth in that massive technopolis. Later sci-fi works since the middle 90's are aware of those limitations, and you all know mining facilities in other planets became a cliche of modern sci-fi.
So sci-fi gives you something that at light of science you think could be possible in time based on this and this and if we do this and this, while fantasy gives you something you probably think you wish it could be possible but there is no way that can happen in the real universe unless there are these divinities, and titans, and fairies and gnomes hidding from us and would grant all those wishes to us.
