One of the reasons, but there's also oversaturation
While yes, we can argue that folks are looking for device consolidation and so those who have tablets and smartphones are starting to upgrade to phones with larger screens to phase out tablets in some aspect, I think the cause of the slowdown is even simpler; we've oversaturated the market.
Most folks who want a tablet, be it iOS, Android or another operating system, have purchased that tablet in the last few years. Unless they need an upgrade or need to replace, there's no need to purchase another tablet. And since these devices are relatively simpler in terms of operation, they don't need as quick a replacement as many laptops, which means the IT buying cycle is much longer. Even so, once they do upgrade their device, they create a secondary market when they sell or trade in their old device. So will we ever see the same tablet shipment forecasts like we did years ago? I don't think so, unless there is a huge technological overhaul, I think it's going to keep a steady single digit for awhile to come.