What are the business links between these two cities to generate the high yields I presume this route needs to survive?
Aviva (formerly Norwich Union) used this route (which operated two or three times a week, usually with an E190) intensively when it used to be run by Flybe. The flight only ran because the aircraft was being positioned in to operate a "bums to the sun flight".
After they dropped it, NU continued to charter 15 - 20 seat aircraft several times a week to commute staff between the two locations.
Of course, there's no guarantee that Aviva would still provide the same number of passengers but there are several other strong business links between the two as well as the two being popular tourist destinations. Under Flybe's pricing policy, the return air fare will almost certainly be cheaper than the train.
The aircraft would otherwise be positioning to and from NWI/EXE to provide the NWI/MAN service, so it makes sense to put passengers on it.