I believe this and market saturation are part of it. There have been improvements in e-readers, but they've been incremental.
For me personally, the biggest inhibitor of getting a new e-reader (or getting the first) is the business model, which has more than a couple of ramifications.
1. You pay a significant fraction of the price for a physical book, and you're only licensing the ebook. Amazon or whoever can smite you and your library whenever they deem necessary and leave you with nothing for your money (if you play by their rules).
2. Competing formats + drm (again, playing by the rules) prevent or impede usage of any e-reader as a universal text reading device.
3. Ebook offering unavailable: new releases - for various reasons - may not have an ebook version offered until some later time.
I would be significantly more interested in buying more ebooks/readers if a physical book + ebook bundle were readily available (at near hardbook price).
From a technology standpoint, I have a 3rd gen Kindle and like it as a tool and really love e-ink and its power efficiency, but I think people would be encouraged to upgrade if these improvements were achieved:
1. near 8.5"x11" size (affordably), for reading textbooks and magazines/papers
2. *some* color - doesn't have to look like Oz, but something please, for reading textbooks and magazines/papers.
3. stylus writing functionality - kind of wishful thinking into the future here, but a feature allowing you to scribble notes into your ebook, or create blank pages to take notes or write out homework.