I'm only including time because of SCAG..I think we can rule out the years to sell part, sales over the first 12 months of the release of the product are down, we are not comparing lifetime sales.
Why that is, not sure, it could be that if you are looking for a new adventure to run, you can now choose between 20 instead of between 5, as they are all still in print, but they could also be not as good as some earlier ones. I was not particularly impressed with the last ones I got and there certainly were complaints about DL, SJ or Shattered Obelisk
But to me the themes of the later adventures don't invoke a "must buy for the feel" feel. You never feel that you need them or are missing out not having them or that they make the work so much easier.
I am in sales by occupation. The WOTC adventure don't upsell well.