A key factor in the fairly rapid decline in operational 330 numbers was (as is often the case with older aircraft) maintenance costs.
Most 330s were operated on either G- or N- registrations and were on the individual operators' approved maintenance schedules which included on-condition maintenance of the PT6-45R engines. This meant that by the time disposal was considered the engines' time since overhaul were way in excess of the manufacturer's schedule. Potential new operators in other countries therefore faced expensive overhauls before the aircraft could have operated. These airframes were not worth the cost of those overhauls which is why so many were parted out or simply scrapped. Shorts had provided residual value guarantees on the majority of airframes to get them financed in the first place. For the reasons above those values were too optimistic, so the British taxpayer ended up owning them towards the end of their useful life.