Well, US distance runners have done fairly well at various points in the past, and there's no reason they can't run just as well now. There are several guys who have run sub-28 10ks recently, for example. The problem, at least this year, seems to be getting those peak performances to take place at a championship meet. I don't think there is a complete lack of talent, or even a lack of willingness to work hard, but maybe some athletes need coaching help and help planning their schedules to peak at the right time. OK, a sub 28 10k isn't going to get you much at a championships, but having a few guys do that would at least be a respectable showing. And if a few guys are there, maybe one will bust through and run sub 27:30 or so and actually contend for a medal.
I'm starting to think of the African domination of distance events as sort of analagous to the USA's traditional domination of the 400m. The US has always owned the 400m. Something about the body structure of the African-American male seems to lend itself to success at 400m. It must be discouraging to runners from other nations. A lot of countries probably see a couple of their guys run 46 seconds and treasure them and try to develop them as their national hopes, whereas in the US a 46 400 doesn't get you squat. But the point is, while the US has dominated and will probably continue to dominate the 400m, people from other countries have been able to break through here and there. Even a few European white guys have done pretty well--remember Thomas Schonlebe winning the WC's, beating Butch Reynolds? The odds may be stacked against them, but that doesn't mean a few here and there can't reach the very top every once in a while. And I think with the right planning, US distance runners can look for the same type of success--maybe they'll never have the incredible depth of Kenya, but they can at least put together a few guys to infiltrate the lead pack.