Adam/David,
Turnberry is only a bit of a long drive if you don't live in Ayrshire ! Which is just another way of saying it is really about the demographics and not the access although obviously the easier/quicker the access then to an extent (probably a small extent in the case of Turnberry) that increases the number of people living within a particular travel time.
If you look at the map of Scotland, the main city for Turnberry is Glasgow. For most golf fans from Edinburgh it is simply too far away. Particularly when the Open is held at Muirfield, St Andrews and Carnoustie so often, which is a much easier commute, why would they bother ? Even Troon is a doddle for Edinburgh folk being motorway/dual carriageway pretty well all the way with the added option of the train. And for Glasgow/west of Scotland golf fans the east coast venues are all accessible to one extent or another although Muirfield is by far the worst if going by car and I very much doubt there is any chance they will lose the Open due to access issues.
Niall
ps. just realised I've been to every Open held at Turnberry which does make me feel old. I do recall the moans about access after the first or second one and promises made about the road being dualled all the way down which obviously never happened. Since then the traffic management techniques have improved and it really wasn't an issue in 2009, particularly compared to Muirfield.