Sotonsean states that the coaches are operated on behalf of the cruise companies. Any flights would presumably be the same - it may be that EZY flights from BFS and GLA are largely prebooked in the same way.
For MAN the advantages of the coaches - the ability of the coach to pick up at various places en route, and avoid a separate coach from SOU to the cruise terminal - may outweigh the faster flight.
What would be interesting would be flights bringing cruise passengers from the continent but Brexit may have ruined that.