i assume it comes and goes in waves. If Marco Langon had run this year, there would have been another American in the top 10. Riley Hough, George Coultie, and Gary Martin might place high, and Rocky Hansen could easily be top 5. Trayvon Kitchen could place top 25 or so, and he'll only get better.
Also, keep in mind that the top runner for Syracuse and one of Wisconsin's runners is Israeli, and Tennessee and Eastern Kentucky's top runner is Moroccan, Mississippi's top runner is either Ugandan or Eritrean, and Valentin Socca of CBU has one of the fastest college PRs in the 5K the NCAA has ever seen and he's Uruguayan.
As for Americans, it's cyclical, they're just having an off-year
Last year, everyone is assuming the Kenyans would sweep, and I believe there were only 2 Kenyans and one Eritrean the entire men's top 15.