There have been some misperceptions about professional cycling on this thread which I'd like to address:
1) Several have claimed that it is minority sport. Well, in the USA it is. But in much of Europe, it is the principal participation sport. Don't forget that the Tour de France is the second most popular sporting event in the world (behind the soccer world cup which falls only every four years), and of the 10 million spectators, most arrive at the roadside by bike. It is also a huge sport in Columbia and other parts of South America, and increasingly Australia.
2) It was claimed that Armstrong's domination implies a weakness in competition. Superficially, this is true but the reality is more interesting. If you look at one day races, or stages in a race like the Tour de France, there is an astounding level of competition. Each Tour de France can potentially throw up over 15 different stage winners. No one wins everything. If a race is largely flat and finishes on a relatively flat surface, Armstrong himself is not in the world's top 100 riders. Armstrong's forte is as a stage racer. Stage races are multi-disciplinary events, with flat stages, time trials (short, long and even uphill) and mountain stages. Imagine riding flat out for 4 to 6 hours a day, twenty days out of twenty two, in all weather. It is so physically tough that anyone fractionally stronger will, averaged out over three weeks, gain an appreciable advantage over the next guy. Armstrong's dominance is a tribute to his will power, self discipline and manic training regime, not the weakness of competition.
3) It is true that he would not be a great runner, but why pick on running? Cycling also involves intricate stategy and team work, which long distance running does not. I'm sure East Africans would potentially shine at cycling, but any triumphant Ethiopean would have to be a master tactician as well. So, please don't think of running and cycling skills as equivalent.
4) Armstrong's success lies in his attitude and not his nationality. I compare him in terms of single mindedness to the great British decathlete Daley Thompson (although Daley tells much better jokes!), who would do extra training on Christmas day because he knew his opponents would be having that day off.
In answer to your question of why can't US runners succeed while Lance can, I'd reply that in Mr Michael Johnson, you had someone who I think was comparable with Armstrong and that such people only come along once in a decade. If that. So perhaps Armstrong is not the most fortuitous frame of reference...