If it was iPad specific in regarding declining sales sure, but tablets all around seem to be down. Explain that?
You make it sound like a riddle. But it's the same reason iPads are declining... same old, same old. But there is a difference. Android tablets for the most part are predominantly sold as media consumption devices and marketed to price conscience consumers. Apple OTOH originally positioned the iPad as a post-PC device, something that would grow past just being a media consumption device into a productivity device. It partnered with IBM to get the business segment, it produced a "Pro" model targeted to graphics professional; none of this has been done in good faith on the Android side.
Also to your point, not all tablets have seen a decline. The Surface Pro 4 (and it shocked me to see this) helped MS tablet sales increase YOY by 29%. I'm not a Surface fan, mostly because I don't like Windows, but it's hard to argue with those numbers that maybe MS has finally made a Surface Pro worth consumer's consideration; something Apple hasn't done with the iPad for years. It's just rehashed the same product with a faster chip, sometimes slimmer, sometimes lighter but nothing to really enhance productivity, like say the Pro's Pencil.