Putting Ostend aside for a moment, the idea of considering Eurostar for a journey from Manchester to Brussels is a complete non-starter except for trainspotters and those terrified of flying. A minimum of three different trains required (including a hideous cross-London tube transfer) even for a city centre to city centre journey. And high rail fares. Alternatively, convenient airline schedules from MAN to BRU or CRL with a local connection. Given those options, Eurostar never enters my thinking.
For Ostend, travel from my home would require a minimum of FIVE different trains including the hideous tube transfer. And many arduous hours of travel. This is exactly why the case for Ostend services from the North and Scotland stand up to scrutiny in a way that London flights do not.
BTW, Ostend is actually one of the closest airports to its city centre that I can think of within Europe. The local bus journey to the rail station is shorter than inter-terminal bus shuttle transfers at some European airports. Connections to the coastal resorts are available via the nearby 'De Lijn' coastal tramway; quick connections to nearby cities including Bruges and Ghent are offered from the co-located bus/rail/tram interchange in Central Ostend (a couple of miles or so from the airport terminal).