Agreed - most regular London travellers I know have been using the LCY only to use up forward bookings made months ago. Any new travel is being booked on FlyBe through LGW.
Speaking to people its a mix of uncertainty over the route and Aer Arann, the service, and the price. Since the smaller aircraft have been put on the route prices seem to have gone up substantially eg booking just made with FlyBe for a trip in the last week of November for £150. Equivalent flights on Aer Arann quoted at £230.
A real shame because before they messed up the service following the Aer Lingus Regional contract I would not have had a word said against their IOM/LCY service.