Just a fer thoughts...
Presumably there is insufficient demand for Royal Mail to operate services from both NQY and EXT to EMA. If the operation was switched to NQY it would surely not save on road transport as mail from east and north Devon would have to be trucked to NQY. I would guess that the business community in and around the Exeter area may produce more mail than that of Cornwall and therefore more trucks may be required (at greater cost) than at present.
There may also be a weather issue involved as well. EXT has a pretty good record weather-wise and accepts far more diversions than it has to turn aircraft away. That would imply the possibility of a more reliable service for Royal Mail than if it operated from NQY.
As well as the Jet2/Channel Express service from EXT to EMA, Titan also operate a B733 nightly to/from STN.