Because the DSA route is using a discounted deal on fees etc set down by DSA/Peel. It's a business deal as opposed to using public money.
The LBA route is using government money to get it going.
What was stopping Flybe getting a good deal at LBA?
Why should my taxpayer money be used to subsidise a flight I will never use when Flybe are starting a similar route down the road with no such requirement for a hand out of public money?
You are missing the point that Flybe can start a route using traditional methods at one airport, but up the road couldn't possibly start the route without a public handout.
What makes LBA-NQY a PSO route but DSA-NQY not?