Although it is true that the time frame means we can't "get" a lot of the jokes, I don't think this is what makes it less popular, because even without getting some of the parody, NA is still far funnier than any of JA's other books, and I think most people would agree. So it isn't the LACK of humour that makes it less popular.

I think it may be the opposite--too much humour. People often do not take seriously a book that is very funny.

But I think one of the other main reasons is it gets no air time. I mean, most people get assigned P&P in school, and are exposed to JA that way. Movies--more than one--have been made from all of the books; but I think NA has only one, and jdging from what I've heard here, it's pretty bad. (But another is coming, yeah!)