Part of it is that it seems that within a year, the good questions were pretty much answered. It's notable that in the first year, especially the first six months, the site was carefully curated and had a friendly feel to it.
In the present SO, answering questions has become a matter of getting your low-quality answer to a low-quality question out extremely quickly or having some pretty obscure technical knowledge (even then, you're going to get karma only for obscure knowledge, not for a quality answer). And, yeah, I suppose you game things by creating a question similar to an existing high karma question (and this stuff also lowers average site quality too since the best answers kind of hide within ten similar questions).
Further, SO ironically turns out to be much useful when you Google than when you ask your own questions - questions beyond a given difficulty go wanting even if you give a high Karma reward. It seems the smart answers just skim for simplistic questions rather than spending time on any hard questions.
It seems like SO is now just kind of a low-quality-question fest because they've got a troove of good answers from their first year of existence.
I mean, one can claim hn has declined but any decline in hn is a thousand times less than the way StackOverflow has tanked as a site (but it's still great for googling all the good answers from yesteryear. When you're doing that, take note of the answer's dates, btw, I believe that it revealing).
