A good question but what other explanation is there ? Ineptitude ? Surely not, this is too calculated.
One factor is that Flybe took over the transatlantic feed from EIR. According to a response on the SEN Facebook from a SEN spokesperson (and here we must suspend disbelief as to the reliability of SEN's
PR) it was Aer Lingus not Stobart/EIR that terminated the agreement in favour of Flybe. This was about the same time that Flybe services from SOU EXT and INV began to offer transatlantic connections via DUB. All part of the same deal - who knows ?
Whether that amounts to a real obligation to maintain SEN-DUB I can't say but I would have thought an immediate withdrawal would have been embarrassing especially as EIR passenger numbers on the route had been steadily growing including a 20% increase in September, the last full month of EIR operation.
Then there is the agreement with SEN/Stobart at which we can only guess. As others have commented, Flybe can now close the route because they have tried it and there is no traffic. Their focus is at LCY not SEN. And now look at CWL.Predatory action against a codeshare supposed partner ?