Perhaps the recruiters are focussed on tighter geographical markets than the whole U.S.
In the UK, recruiters can just have one London office and claim to be national because:
a) 99% of the consultancies are based in the South East
b) it's possible to get to London and back in a day for most candidates
c) the UK is regulated as one labour market so a firm doesn't have to worry about customising its processes for each assignment
I don't believe the same factors hold true in the US. You may have more luck searching for New York executive search firms, or Bay Area firms, etc.