I think it is more about brand/name recognition and book store capability than technology. Whether you like Kindles or not, in the USA the name "Kindle" has become synonymous with eBook reader devices much like the name "Kleenex" has with facial tissues. It also makes no difference whether ePub or mobi/KF8 is the better eBook format. Most buyers of eBooks probably don't know a thing about formats nor do they care to. In the USA market Amazon sells more eBooks than all the other eBook stores combined; about twice as many according to Wallstreet analysts. That is unlikely to change any time soon given the lackluster competition which Amazon faces.
In reality, all the current eInk reader devices are fairly close in specs, price, quality, features, etc. The same can be said for the 7 inch Android tablets. One company releases new readers and for a brief moment in time perhaps they have a slight technological/feature edge, but within a few months other companies either release comparable readers are leapfrog them with slightly better ones. I doubt the average eBook consumer really knows which readers are better than others, nor once again do they really care.