The biggest problem, according to one panelist, was a lack of historical vision in children today. He could read stuff from decades earlier when he was a child -- in fact, the first twelve SF books he had read were all written before he was born -- and make allowances for the fact that it had been written earlier and from a different point of view. Then, he had grown up swathed in historical stuff.
Some tried to prevet the stuff, and discovered that some of it was not what they remembered, but even stuff that held up was not necessarily enough to hold the attention of kids.
One of the youngest fans in the room pointed out that much of the stuff that grown-ups find problematic went right over her head when she was a child.
One panelist pointed out that in the sixties and so the youngsters could tell those who had grown up on E. E. "Doc" Smith that SF had improved. He didn't think that could be said about SF nowadays versus the sixties.
Publishing changes did not help. In the sixties, a midlist author could not only have a collection of short stories published, it could stay in print for over a decade. You see, at the point a SF cover meant that your work would sell about 50,000 copies. That meant a lot of experimental and bold stuff could get published. Then Foundation's Edge came out, all the fans who'd accumulated over the decades to Foundation all bought it at once, and publishers discovered you could have SF bestsellers. Got a lot harder to get in print and stay in print and publish new works then.