I don't know how anyone could add such a large aircraft to a lightly populated airport like Sumburgh, especially with another company already established with smaller unfilled aircraft.
Did they think that the oil companies would use it? There are good reasons why the oil companies use charter flights. They can regulate the forwarding of people to Shetland if weather conditions, or other delays, are causing them logistical problems. They can also get their home-bound crews out of Shetland whenever their chopper lands from offshore.
I'm going back a few years but I doubt if things have changed much. Many of the charters would not leave Aberdeen if the helicopters were not cleared to fly out to the platforms. There was only limited accommodation available so sending crews up there on spec could mean a lot of expensive flying up and down.