I think one of the reasons may be that EMA's catchment area can be described as an X mile radius....360 degrees.
Manston is probably restricted to the West. No towns North, South or East. It has a lot going for it, Good Terminal relatively new, Good air traffic, but there is no real rail service, and the Motorway stops at Faversham, and passenger perception is "Where"?? Manston has always been a "nearly" airport, and I, for one, would be delighted if it made a success of it, but I certainly would never put a shilling on it.