Well first we have LH's huge hub @ FRA and the only other 'hub' operation from SOU is a KLM codeshare on the Flybe AMS flights, whereas the other airports you mention all have links to at least 2 major hubs by other airlines (BA, KL, AF, SN etc). I also think people are generally more likely to book a connecting flight if it is all on the same airline rather than just a codeshare.
Secondly there is obviously a fair amount of O&D between the two cities as the route has lasted quite long and I believe it was said on the Flybe thread by hampshireandy that it always had good loads. And this was despite the fact that there was only 1 flight per day (0 on Sat) and it was poorly timed for business traveller's who would ideally need an early morning and evening flight to allow for a working day.
Two flights per day also help to improve connectivity.
They are the main reasons why I feel a LH route from SOU would work if it were the right frequency's at the right times.
Also not relevant to this post but for anyone interested the BA Firefly A319 is due in at 17:45 tomorrow to take the Olympic flame to the channel islands.
Last edited by adfly; 13th Jul 2012 at 14:07.