Laptop means an emissive display, which generally results in excessive brightness in lower light scenarios and inadequate contrast in very bright ones, because it needs to power through the ambient light. Epaper is way easier to read because it inherently matches the lighting of your environment (or you can use a front light to boost it slightly in the dark) by being reflective instead. There are interesting efforts at reflective LCD screens, but they're even more expensive and limited to monitors and TVs for the most part. For text based content, eink and other epaper devices read like actual paper, and you can't match that with other display tech currently. The display is most of the cost of those devices, though, because they're still pretty low volume and hard to manufacture.

I'm not sure the distinction you're making with "big phone". The bigger ones support pens for you to write on them, and it feels similar to using my iPad to read, just without animations and with a more paper like display that doesn't get blown out in the sun. (The current version would be the tab x, just to clarify.) I think Apple's tablet experience is a lot better than android's, and there are a bunch of apps that I like that aren't on Android, but I wouldn't say it doesn't feel like a tablet.