You are right in that she doesn't dwell much upon the lower classes of people- only as much as is necessary to give illumination to her story. However, it might be worth noting that she actually pays a similar disregard to the upper end of the social spectrum also. Royalty are not mentioned at all, and the aristocracy as a whole equally marginalised. We have the Earl of ________ as a passing mention in P&P, and Viscountess Dalrymple as similarly passing mention in P. Lord Orville and co in the Watsons never really get off the ground, either. Who is left? Lady Catherine, a has-been aristo who is no longer the daughter of an earl, and not even the wife of a baronet any more, has to reinforce her social position by dominating anyone and everyone in sight. Sir Walter, in the very lowest and shakiest rung of the aristocratic ladder, has no money, no power that he's able to use (although he has the "gentry" qualification of being an MP, possibly )and is so busy trying to pretend that he is more than he is that he has no time for real life. Sir Thomas, squarely in the gentry with his MP satus, doesn't have any appreciable contact with any of the arsitocracy except yet another younger son of an Earl in Mr Yates, and Colonel Fitz- another moneyless younger son, doesn't actually do much either. It seems to me that JA tried, as far as was possible, to keep the aristocracy, and especially the upper aristocracy, out of her writings almost as much as she did the servants.

Could it be that she knew her "market" was with the gentry, as much as the fact that she knew the landed gentry better than any of the (many) other classes in her life?