Sci fi also changed a lot during the years. Initially it's been technical fiction, and nowadays it's more social fiction. Evolution of society, and problems if it develops in bad ways, are of interest for many people, while the technological fiction was (and I believe still is), only of interest to a few.
The genre of sci fi definitely widened and opened up to themes that more people are interested in, and therefore it became more mainstream.
Sometimes I see the technical old-style sci fi now labeled as "hardcore", which might indicate that tere are still parts of sci fi which only appeal to small groups, and definitely are not mainstream.
Having said that, my personal impression is that Star Trek is not very sci fi, it's just the traditional adventure novel put a few years ahead, and the Star Wars universe is much the same, even with a bigger touch of fairy tale elements. But people call those sci fi, too, and so definitely it became more mainstream.
Fantasy ... you know there always have been fairy tales. There always have beens sagas and myths. Fantasy is just the modern way to take these a step further. So I'd almost say, these were mainstream all the time and still are
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