You just have to look at a map to see the problem that Newcastle has.
It is a relatively isolated connurbation so fishing in a relatively small pond for customers. You might attract customers from outside the northeast if you can offer destinations which aren't served by EDI, LBA, MAN but as you are starting from a smaller base that is unlikely - it is more likely that NCL is leaking customers to these largerairports. Also you'll attract people on price - hence the long standing school summer holidays switcheroo between NCL, GLA, EDI.
The north east is also relatively poor area compared to the surrounding area, so there is less propensity to fly. It's also less diverse than other regions, so there isn't the VFR demand from various diaspora.
New routes might reduce the leakage to other airports but relatively speaking NCL runs the risk of slicing up the cake into smaller pieces rather than increasing the size of the cake.