the weather is not up to the heat of Mediterranean beaches
Exactly! This is one of its biggest attractions. Not everyone wants to be cooked.
Bruges can be reached fairly easily by train from Brussels airport
Belgium is not a large country, but Zaventem-Brugge is around three hours with a change of train; OST Airport-Brugge is about 30 minutes with short bus or taxi ride between airport and station.
Ghent can be reached even more easily from Brussels airport
No, Gent St Pieters is reached far more easily from Oostende than from Zaventem.
Is there sufficient demand to Ostend airport as distinct from Brussels ?
From a region such as NW England which is not served by the Channel Tunnel,
yes. Bruges is a big deal. The other attractions which I itemised earlier play a role too. The Channel Tunnel and ferries to Calais make air services unviable from the South-East.