Well, for one, it's a serious annoyance to fill out the customs form. You have to do it exactly right, or do it over. They are seriously anal about it, at least in my post office.
Plus, I think a lot of sellers aren't willing to deal with the almost inevitable "it costs HOW much?" question.
Plus the necessity of more careful packaging, the total inability to track it (so there is no chain of custody), etc. put a lot of sellers off.
Not saying it's right, but then, I've often dealt with the issue in the reverse direction. Getting a Sony-Ericsson P990i was an exercise in frustration, as nobody was willing to ship it to me, even for $100 in shipping. Apparently sending in to the US is a giant hassle.
Oh, and technically, the USPS is a government run, for profit organization. Or did you mean in France it isn't state owned?