I think predicted weather patterns has something to do with it.
Also, the precise position of the launch site in relation to sattelites (which are not always geostationary) means only certain routes and time windows are possible for fuel economy, and to make sure the shuttle can make it back to base.
Remember, when doing big astrodynamic (my new word) calculations, little errors quickly multiply, and you wouldn't want to launch late and end up on the wrong side of the earth for re-entry because of a little delay!
And there's the American fixation with accuracy, and also because it's so cool to go 5... 4... 3... 2... 1...
BLAST OFF! WHEEE!