Foreign nationals don't meet the "Unquestioning loyalty to the US" criterion for security clearance, meaning it is much less likely they might collaborate willingly with the US. They're also less exposed to pressure from the US Government, especially if they are not resident/GC holders.
I personally think it is quite paranoid to assume that any US resident or national cryptographer is in cahoots with the NSA but if we are strictly considering risks, then this process is open to scrutiny by world-wide participants, many of whom may not sit on a vulnerability at the behest of the US GOV - actually, for many of them, finding such a vulnerability would likely mean a top tier paper at CRYPTO/EUROCRYPT probably with best paper awards and all the plaudits that make an academic career for life.