Could the setting up of a Manchester base be a tactical move related to start-up of flights from Doncaster/Sheffield? I assume that Jet2 have studied the catchment area for LBA and found that few travellers come across the Pennines from Greater Man.(as mentioned above) but I bet the LBA catchment area spreads well into South and East Yorkshire. DSA is bound to capture alot of this traffic and hence Jet2's business at LBA could be heavily eroded. Therefore they're setting up a base at MAN to become less reliant on LBA.
I flew from LBA several times between 2000 and 2002 and the major problem(even today) imo are the transport links. The journey to the airport from most directions is painful. I know people in parts of West Yorkshire who can get to MAN quicker than LBA solely due to the poor road links. In an area of the UK surrounded by other airports(with far superior road access), this will always limit the demand/appeal at LBA.