When Simon calls in the robbery, he gives the location as being on Broadway in the upper 90's. This is a couple of miles from the theater and, even if he were going to the Secret Service field office (as CJ notes later), he'd be going the wrong way; the field office was moved to Brooklyn after the September 11 attacks.
Toby decides to send President Bartlet's motorcade to tie up traffic in an effort to prevent Ritchie from getting to the theatre. A motorcade would never leave the President's location, in case something happened which required a quick evacuation of the President. Additionally, it is highly unlikely that the White House Communications Director could direct the movement of a Presidential motorcade.