Electronic content doesn't face the same over-supply possibility that physical content faces. For example, let's say that you buy 1M copies of Book X for $2 each to sell. Well, a year later only 250,000 have sold at $4 and the rest are just sitting in your warehouse. You don't even care if you sell them at cost now (or maybe below cost). You've spent $2M and taken in $1M and have 750k copies left. If you sell the remaining 750k copies at $2 a piece, you make money ($2.5M with costs of $2M). In fact, you could sell them for less than $2 if that would make them move to recoup the remaining $1M. So, there are supply issues that make the price of physical items more variable. With electronic items, you just get the price. Since there isn't a limited supply, you can have unlimited copies on day 1. They don't over or under buy, they transmit exactly the number of copies purchased.
Likewise, as a physical book ages, competition goes up. You might be given it once your friend is done with it or be sold it used. That doesn't happen with Kindle books (or not as much, I know the Kindle has some limited lending capability) and so the price stays stable since the level of competition is stable.
Many prices face competition with many sources. With eBooks, the number of sources declines, but electronic resources are cheap and so it's stays at a point that we merely complain about rather than searching for alternatives. So, losing first-sale rights means losing the cheaply available second-hand books that provide competition that helps to keep prices low.
Now, I'm not blind to the other side: a used book is less valuable than a new book. It's creased, written in, whatnot. A used Kindle book is identical to a new Kindle book. So, it might be disastrous for publishers to have all users be able to re-sell in an unlimited fashion because there's no aging of the physical thing like there is with physical volumes. But that's something to think about for another time.
