I don’t think older SF is being “forgotten” and I don't think that the youth of today should know better. I do think that readers want different things, especially in terms of character, and older SF may not suit their requirements.
Firstly, a lot of older SF, while remembered fondly by those who encountered it the first time around, doesn’t hold up very well. Not just, as Pyan says, because its technological innovations are old hat, but because it’s not very well written, or is written in such a way that only the SF parts of it are especially strong. When I read Dune, I was surprised at the depth of the characters and the setting compared to other classic SF (likewise Titus Groan compared to other fantasy, but that’s a less fair comparison). Compared to Dune, something like, say, Foundation, whilst not badly written, feels rather flat. It’s clearly the 1950s transposed to a future setting, which was fine back then, but feels clunky now. That’s not to say that it’s bad, but that it fulfills different criteria. I think it’s unrealistic to expect modern readers to get the same thing out of Clarke, Asimov or Heinlein as people did when they first came out.
The focus of SF – at least the edgy sort that wins awards – seems to have changed. It appears to be less about predicting tech as exploring modern-world experiences in SF terms. The question seems to be less “What would it be like if we had the power to do X?” or “What will X be like in the future?” as “What is it like to be X kind of person in SF terms?” A tech-based SF only really needs characters to work the tech and repair it. The tech is, in a way, the most important character. A character-based SF requires deeper characterization, but it can entail more navel-gazing.
Some of the trappings of older SF – the psychedelic drug-taking of PK Dick, or the Japanese Zaibatsus of early Cyberpunk – feel dated. While the core of those styles – Dick’s sense of unreality and alienation, and Gibson’s noirish attitude – remain applicable, it’s harder to get into those authors where part of their setting feels retro at best.
However, really incisive books – not just books that predict a technological advance or instill a sense of wonder, but those that say something really important about being human – will survive. There are loads of things wrong with 1984, but it is still the best analysis of the psychology of a dictatorship that I have ever read, and I think it’s that which will make it relevant in the future.