I think of "pure" sci fi as being the same as "hard" sci fi - that just means the writer accepts the limitation of adhering to plausible science rather than bringing in the more fanciful notions that are a shortcut to writing compelling drama that people can relate to - Earthlike planets around every corner, alien species mainly being humanoids, everyone conveniently speaks English or uses a flawless universal translator that never gets cultural nuances wrong, even in the case of completely unfamilar cultures and that sort of thing.
No, that's not a fair definition, because it excludes a vast cross-section of science fiction. Hard SF is merely a subgenre, and speaking as a hard SF writer myself, I would never be so arrogant as to claim that my subgenre is the only one that qualifies as "pure." That would be an insult to the likes of Bradbury, Sturgeon, Ellison, LeGuin, and countless other greats. Heck, it would exclude Shelley's
Frankenstein, which is generally considered
the first work of science fiction. It would exclude the entire canon of H.G. Wells, and it would be outrageous to claim that H.G. Wells didn't write science fiction. So no. "Hard" SF is not "purer" than other branches of SF.
Science fiction, as I said, is fiction that is grounded in a "What if?" question, a speculative premise based on scientific or technological progress. It doesn't have to be strictly rigorous science; that's just one approach, one style. It just has to be a story that couldn't be told without the speculative element at its core.
The demands of fiction writing will be the same regardless of whether the sci fi is hard or not. You'll still need relatable characters and a premise that the audience cares about seeing a resolution to.
As I've said before, that's less true in hard SF than it is in other genres, because a lot of the audience base for hard SF can be satisfied just by the science and worldbuilding and forgive weak characterization. Of course, these days, that doesn't go over as well as it did in the past, and most modern hard SF writers try to balance the "literary" stuff like characterization and writing style with the more technical stuff.
Hard sci fi just removes a lot of the tricks from the writers' bag, and because there's a limited audience that will appreciate the restraint, why should writers do things the hard way?
I think that hard SF adds more than it removes. There are so many wondrous possibilities that the real universe offers, possibilities that are ignored by film and TV writers who don't know about them. Just to cite one example, "soft SF" tends to give us aliens that are just humanoids or energy beings or familiar tropes like that, whereas a grounding in biology, physics, and other sciences can allow you to design far more imaginative and extraordinary aliens, like Niven's puppeteers or Egan's Wang carpets. And studying real planetary science can produce ideas for all sorts of extraordinary environments that nobody would've ever thought of without science to point the way. Nobody ever imagined that Europa or Enceladus could have an ocean beneath an icy crust until real science showed us it was possible. Science doesn't limit the imagination, it guides it to new places.
And "why should writers do things the hard way?" Because that's what good writers do. They don't look for shortcuts and settle for routine approaches. Good writers aren't afraid to challenge themselves. Working within limits is a good way to do that. When Robert Hewitt Wolfe developed
Andromeda, he made a point of refusing to use FTL communication, teleportation, forcefields, tractor beams, and other hoary old tropes, precisely because those were easy outs, convenient and familiar fixes to story problems. By embracing more of a hard-SF approach, he challenged his writing staff to come up with fresh solutions and rise above convention and cliche. And it worked, making the show fresh and interesting, until Robert got fired and replaced with a hack who just regurgitated all the familiar cliches of sci-fi TV and robbed the show of any intelligence or originality.
The what-if aspect of sci fi is the other "burden" that is hard to reconcile with the mass market. There are some attempts at what-if sci fi on TV now, struggling to survive or not surviving - Journeyman, Dollhouse, Virtuality and Caprica are all in the what-if category, but to audiences who want some basic premise they are familiar with, like the war story or the cop show, it can be the so-what category.
On the other hand,
Galactica was a "What if?" premise: "What if human civilization were destroyed and the last survivors fled as refugees?" "What if we created an artificial race that was just as human as we were?"
Heroes is a take on "What if humans had super powers?" and it was quite successful initially; its decline is not because of its premise, but because of a deterioration in the quality of the exploration of that premise.