Why would anyone be surprised by this?
Once someone has a product that provides a function, they will only get a new product if there is some very compelling feature in the new product.
Gone are times of yore when a year of hardware advancement makes a computer that is appreciably better than what came before it.
There is typically no value in going from a plenty fast enough dual core to a faster than I can appreciate quad core. No surprise then that demand stagnates.
I have a Kindle that is a couple of years old. It works fine. I've never maxed out the storage, so doubling or octupling the storage is of no value to me. The battery life is already absurdly long. Doubling the battery life would be of no value to me. I don't want more features (indeed the Kindle already has too many features that detract from its core function - email on a Kindle?? WTF).
The same goes for computers (laptops and desktops), phones, phablets, .... There are very few people who will get a lift from upgrading because the computer is already faster than it needs to be (for most people).
I used to buy a new laptop every 18 months (business expense: I like having something fast for compiling etc). However the laptop I bought 4 years back is not much slower than the laptop I bought 2 years ago and I see no reason to buy another.