Blanche wrote:

Neither the White House nor Pelosi nor her staff requested the
C-32 (USAF version of the 757). The House Sgt at Arms requested
an aircraft that can travel non-stop for security reasons. This
has become standard practice in the past 6 years.

http://www.cnn.com/2007/POLITICS/02/...ane/index.html

If that's the case, than the C-20 would have fit the bill just fine.
Apparently though a Gulfstream is just too damned small.