Smartphones are certainly playing a big part. But I think the manufactures are failing to deliver quality tablets that people would want.
I think Apple has been doing a great job. But iPad sales are slowing down and I think part of that is just because the iPads are so freaking good, that people can go two or even three years with their current iPads now before feeling the need to upgrade. And there are people that just don't like Apple or iOS or iTunes, so stay away from them.
And unfortunately outside of the iPads, there hasn't been any consistency and very little quality. Most manufactures have shifted their focus away from premium tablets to sub $200 pieces of garbage. Samsung released some semi-premium tablets last year, but cut corners on the processor. While ironically there were many $150 tablets with Atoms inside them outperforming Samsung's $500 flagship offers with Exynos inside.
And then we get the HTC built Nexus 9, with premium pricing and corner cutting build quality.
The state of Android tablets makes me want to cry. Nobody wants to buy this crap.
Windows tablets and 2-in-1s are getting more interesting though. The Asus T200, T300 Chi, Aspire Switch 10 and 11, Surface 3, are all pretty decent tablets that become laptops, pretty good performance from the Atom processors now. They are getting pretty light (but still not quite iPad/Android tablet light yet). If Windows 10 can attract developers, then I think we might see a surge in these 2-in-1 devices.