Because it's an airport people want to go to and what it offers?
Remember Location, Location, Location is everything and booking agents have the choice of how many operators now from LTN? Don't know the exact figures but don't Hanger8 and LEA amongst others have significant operations now yet based somewhere else?
If LTN had the space we most probably have facilities for Jet Aviation, Signature always leads due to its depth of international operations, a lot of investment in US at the moment and more to come in Far and Middle East.