I can vaguely remember that bus from when I did some work on Doxford Park.
The problem with that bus, was that it missed out a big chunk of the villages and towns along the way. It was great if you lived on the route and needed to be at Doxford Park, starting /finishing a shift when the bus was due, but if you didn't or had to travel to get to its route to get it (on top of paying for the crazy fare and any connecting fares)...
Suppose it is finding the right balance between speed and ensuring you are collecting enough passengers on the way.

Mind, it just seems so short sighted with the X7 - it is a service which has a lot of potential. It is competing with regular, established routes between Sunderland and Seaham and again between Peterlee and Middlesbrough, so surely has to make the most of the destinations in-between, like the ones suggested.