The reason it's trending right now is because it's a convenient price, simple to use and making people read more because of its size. If it helps people cut down on phone use then it's a good product.

The build quality could be better, but there doesn't exist a similar product with better build quality.

Can you build a DIY version that's significantly cheaper?

Well, this was my thinking for getting one, we shall see when it arrives :P

Edit: Also you can choose to install a nice open source community firmware