Two of the factors which contributed to its demise were its lack of controlled airspace and secondly its lack of radar to provide some level of protection to its arrivals and departures while in Class G airspace.
A number of airproxes which occurred with Shefield traffic persuaded carriers that they could not adequately mitigate the hazards faced when operating in Class G without radar cover.
The CAA requires each carrier considering Class G operations to carry out a risk assessment to show that the level of risk can be reduced to as low as reasonably practicable. Controlled airspace would have helped, but was not attainable at the time due to the relatively few air transport movements and passenger throughput. Bit of a "checken and egg" situation.
Radar could have solved it, but appears to have been beyond the reach of investment funds available.