Originally Posted by
chevvron
The answer could be NIMBYs.
A couple of years ago, a new departure route for southbound traffic was trialled so that instead of Heathrow departures going over the Woking area, they went over Ascot instead.
I'm not sure that answers the OP's question about why the NPRs/SIDs were put where they were in the 1960s, though it does illustrate the difficulties faced by any proposal to alter them, even on a temporary basis
There were widespread complaints from the Ascot/Bagshot/Windlesham area and the routes reverted to the original ones.
It was always planned that the trial would only last for 5 months, from August 2014 to January 2015, before reverting to the original MID SID, but such was the public opposition to the trial that it was terminated two months early, in November 2014.