I think it's because e-reader screens are made on a glass substrate, and a large, thin glass screen in a large, thin, plastic device frame is going to be difficult to reinforce so that it doesn't break. There have been a couple of attempts to manufacture plastic-backed e-reader screens – I recall Plastic Logic from Cambridge a few years back, who were going to make an A4 e-reader but struggled to scale up the technology, and shifted to licensing.

E-Ink have just recently got their technology into production, called Mobius, but they haven't yet caught on in retail devices yet (given the market is mostly a walled garden quasi-monopoly, in practice I mean Amazon haven't used them in Kindles yet). Hopefully when the technology becomes mainstream it will remove some constraints on e-readers, and they'll become thinner, more robust, and easier to produce in large sizes. And hopefully alongside that we'll also get some competition in the e-reader market, and devices which are more open.