Why don't Harry, Ron, notice some of the "new" topics introduced in the later books?

Question

Throughout the Harry Potter series, several new topics, and ideas are introduced in each new book. For example, in the third year, J.K. Rowling reveals several new pieces of information, who Sirius Black is, and the Hogsmeade visits.

Third years are allowed to visit the village of Hogsmeade on certain weekends. Please give the enclosed permission to your parent or guardian to sign.

Also,

Stan had unfurled a copy of the Daily Prophet and was now reading with his tongue between his teeth. A large photograph of a sunken-faced man with long, matted hair blinked slowly at Harry from the front page. He looked strangely familiar. "That man!" Harry said, forgetting his troubles for a moment. "He was on the Muggle news!" Stan turned to the front page and chuckled. "Sirius Black," he said nodding...

Harry, Ron, and Hermione do not hear anything about these two things when they are in their first and second years at Hogwarts. This seems very unlikely, as Fred and George would have told Ron about Hogsmeade. Also, Harry would have heard about Sirius from many people, owing to his supposed connection with Voldemort. Why don't Harry, Ron, and Hermione notice any of these things in their first and second years, why don't they hear about them?

Answer 1

There is a principle in writing called the "Law of Conservation of Detail" which holds that anything in the text should only be discussed if it is relevant to the story at hand. Adding extraneous detail on things that are not immediately relevant can confuse the reader, bog down the pace of the text, or just waste time. Instead, the text should dedicate the most words to the most important things and gloss over or entirely skip details that aren't relevant. In this case, it's likely that the characters did know about the Hogsmeade visits from school announcements or personal connections, but since they weren't going to be attending these events until several years later the characters didn't pay much attention and the text did not mention them in detail. This is just a basic principle of good storytelling.

Regarding the Sirius Black example, Harry was only 11 years old in his first year. The fact that the older people in his life did not talk about the man who was presumed to have murdered his parents is totally understandable. It's only when he escapes and presents a possible danger that Arthur Weasley informs him. Again, it's a matter of just not being relevant to Harry.

Answer 2

First, Harry is at least aware that Hogsmeade exists and that the faculty sometimes visit - it's brought up in Philosophers Stone, just not by name (though the Hogs Head pub is).

"Won it," said Hagrid. "Las' night. I was down in the village havin' a few drinks an' got into a game o' cards with a stranger. Think he was quite glad ter get rid of it, ter be honest."

"It's not that unusual, yeh get a lot o' funny folk in the Hog's Head -- that's the pub down in the village."

It is unclear whether Harry knew that students could visit, but he clearly knows Hogsmeade by name; when he reads the letter, his reaction is more focussed around getting the permission slip signed (either he didn't know they could visit OR he assumed they didn't need permission from guardians).

Harry pulled out the Hogsmeade permission form and looked at it, no longer grinning. It would be wonderful to visit Hogsmeade on weekends; he knew it was an entirely wizarding village, and he had never set foot there. But how on earth was he going to persuade Uncle Vernon or Aunt Petunia to sign the form?

Secondly - a broad answer would be that Ron does know this stuff (Hogsmeade anyway, generally only the adults know about Sirius), but doesn't mention it to Harry because he assumes the others already know (sometimes it's hard for him to remember how much of their lives are different from his, and what he thinks of as "basic knowledge" doesn't apply to them).

I think the best example is in book seven, when discussing Wizarding fairy tales.

"And as for this book." Said Hermione, "The Tales of Beedle the Bard ... I've never even heard of them!"

"You've never heard of The Tales of Beedle the Bard?" said Ron incredulously. "You're kidding, right?"

"No, I'm not," said Hermione in surprise. "Do you know them then?"

"Well, of course I do!" Harry looked up, diverted. The circumstance of Ron having read a book that Hermione had not was unprecedented. Ron, however, looked bemused by their surprise.

"Oh come on! All the old kids' stories are supposed to be Beedle's aren't they? 'The Fountain of Fair Fortune' ... 'The Wizard and the Hopping Pot'... 'Babbitty Rabbitty and her Cackling Stump'..."

"Excuse me?" said Hermione giggling. "What was the last one?"

"Come off it!" said Ron, looking in disbelief from Harry to Hermione. "You must've heard of Babbitty Rabbitty -"

"Ron, you know full well Harry and I were brought up by Muggles!" said Hermione. "We didn't hear stories like that when we were little, we heard 'Snow White and the Seven Dwarves' and 'Cinderella' -"

"What's that, an illness?" asked Ron.

So sometimes he doesn't bring up stuff unless Harry asks or is visibly stumped. Hermione has a similar, though less pronounced problem - she's extremely diligent in her studies, and assumes everyone is else is just as neurotic and prepared as her - as evidenced more than once with the recurring joke about Hogwarts: A History.

"Honestly, am I the only person who's ever bothered to read Hogwarts, A History?" said Hermione crossly to Harry and Ron.

So, at least for Hogsmeade, it's likely that one, both, or all three knew. However, since it wasn't relevant to the present, and they assumed everyone knew about it, they didn't bring it up.