Just guessing but I would suspect economics came into it. The Shuttle launch pads were originally built for the Saturn 5/Apollo rockets and modified for the Shuttle launches. The ramp up to the pads was laid out for the Saturn 5 with no sharp bends. The Saturn 5 I don't think had quite the same aerodynamic limitations as the Shuttle+boosters+external tank. Having seen the structures I'd hate to think what it would cost to re-orientate the pads.
There are I believe at least three reason why they take off to the east.
1) Taking off to the west over the Orlando Theme parks is not good public relations.
2) I understand that taking off heading east as near to the equator as possible they gain some speed from the earths rotation- less fuel required.
3) Obviously it is easier to drop off the unwanted bits into the atlantic and makes recovery of the boosters easier.
Someone can probably correct me, but I think NASA have already tried rolling again later in the flight to put the Shuttle on top of the tank but I didn't understand why?
Any other thoughts?
[This message has been edited by Mike Echo (edited 05 December 2000).]