It's always going to be a small. expensive operation - at the end of the day there is very limited accomdation in the Isles and, although it has a better climate than most of the UK, it can't compete with the Canaries or Spain for the sun 'n sand crowd. There is nothing special historically or culturally (cp somewhere like Malta) to attract that crowd
Newquay, which has access to a lot more people and land area struggles to maintain any air service at all.
Th Scilly's are a very small niche holiday market - it will never sustain a lot of competition so prices will be high unless the local Govt. wades in with a substantial subsidy