Most of the people going to London from Middlesbrough will use the train to change at Eaglescliffe and leave their car near the station in Middlesbrough. Can't see people wanting to drive to Yarm, which has a smaller car park than Middlesbrough, when they could just get the train to Darlington and an EC from there.
I dunno. People are very car centric, and I know a number of (business type) people who live in and around Bradford, but don't get trains from Bradford, Shipley or Leeds to London, precisely because they don't want the hassle (as they see it) of changing. Instead they prefer to drive to Wakefield and use it as a kind of West Yorkshire Parkway. Certainly the amount of business done by Wakefield Westgate station is out of all proportion to the relatively small size of the city. The rationale seems to be if I've got in my car to drive to Shipley (or Middlesbrough), I might as well stay in it to a point (Wakefield, Yarm/Eaglescliffe or Darlington) where I can get one train (and save ching on the fare because it's nearer the destination). This applies less if they live close enough to a station to walk and therefore the car doesn't come into the equation.
I certainly see that this might be an issue for the new GC service from Bradford to London, due to the total absence of parking at Bradford Interchange. At least the EC service calls at Shipley, which is a sort of Bradford/Aire Valley Parkway.
If you take a step back and look at things through a normal's eyes, behaviour that seems rational to enthusiasts may well be motivated by a desire to get as much Pacer (or whatever) mileage as possible!