Since the story is (mostly) told from Lizzy's POV, descriptions of Longbourn, Meryton, etc. would be out of place. Why would Lizzy be noticing the size of Aunt Philipps's rooms, for example, as Mr. Collins does? Or the furniture of her own home? Unless something has changed, or a new character brings his perspective, such descriptions would take us too much out of the story. It would make it clear that it was a story with the author conscious of an audience--which JA does not do IMO in P&P as she did in NA. But Lizzy's seeing Pemberley for the first time is pivotal to the story, so it is described in detail--not in a few paragraphs lumped together, but as Lizzy might naturally encounter it and think about it.