Looking at the timetable, the Metrocentre-Morpeth shuttle calling at Cramlington runs until 1844. Presumably as there will be fewer people travelling in the evenings it is not considered worth the cost of running it later than that. Morpeth gets a number of calls by longer-distance services through the evening but Cramlington is not really busy enough to justify these and the platforms are probably too short anyway. In drawing up the franchise specification somebody probably decided that the last train to Cramlington and Morpeth* should be later than this, so a train from Hexham is extended there at 2200 which would probably otherwise just finish its day and go to the depot.
*There is a later long distance call at Morpeth but only on Fridays.