I think Arriva wanted a faster journey bus service for their passengers  between Peterlee and Sunderland even though their not much in journey time as using the A19 in a similar line to the coast road. when I was on the X21 Tuesday gone to Sunderland, their was a solo on service 23 passing at grangetown southbound then a few minutes down the road their was a Arriva streetlite not in service heading southbound towards grangetown.

I used to get the X7 regular which got to Sunderland at 08:30, used to get the bus from Peterlee bus station, the bus arrived almost full, 2 thirds get off, and the bus fills up again, with passengers getting off at seaham industrial estate and the main stop outside Byron place with the same daily passengers getting on to Sunderland, but since Arriva introduced the east durham triple ticket and also reducing the 2 zones ticket to 4.90 saves me money as I can use my local arriva bus to connect with the X21, as GNE wont serve the south side of burnhope Way. only downside is theres not much to do after Sunderland if ive got the day ticket off arriva, but theres more to do if I get the GNE red and turq zone day ticket, but usually I purchase a explorer if im using Arriva, GNE and stagecoach.