To get from PIK to Edinburgh City Centre takes:-
at least a 1.5 hour by car (with added car hire costs) or;
at least 2 hours by train with at least one change and a trek between Glasgow Central and Queen Street
There are also two routes from Glasgow Central to Edinburgh - via Shotts (which I believe serves every town in along the route and takes ages and the London King's Cross train which operates via Carstairs and takes around an hour. I use both Prestwick and Glasgow several times per year and I can't help but notice the amount of passengers at Prestwick from Edinburgh and even as far away as Dundee going to Stansted among other places. Personally if I lived on the east coast I'd rather fly from an airport nearer to home unless it was going to be a lot more expensive to do so.