Travel agencies expect a commision from each holiday they sell and I suppose Kent Escapes are no different, if they aren't getting a decent cut, they won't headline Manston as the place to fly from.
Alternatively, Manston could do what many other regional airports do and set up their own travel agency, but this costs a fortune in start up and in annual running costs, takes a couple of years before they establish themselves and they have to cope with cut throat competition in the area.
Perhaps Manston will just have to take the plunge and decide which is more cost effective - reduce their own profit margin and pay the travel agent the going rate or set up their own.