I don't know, but I guess that with the notoriously long taxiing times at Schiphol, it could well be that the block times for services between BHX / MAN and Rotterdam could be a deal less, same also for the transfer from airside to land side that RTM. The timings from both BHX and MAN are very good for business too.
I don't know that much about public transport from RTM to the rest of the southern Netherlands, but I am guessing that the market FlyBe are going for is business traffic to cities like Breda, Den Haag, Den Bosch, and even Antwerp. With the latter, a twice daily service will prove more attractive to business travelers than the 4 x weekly VLM.