Weather is the big factor with the mainland to Scillies transfer - Lands End is up on a hill and Penzance has no instrument approach at the mainland end and at St Marys you can't get below 860' QNH on the RNP approaches to an airfield that is 357' amsl.
Tresco at SL would be better but there are no instrument procedures there.
Advection fog is the big enemy in the SW. Holding div fuel just means fewer pax and less revenue.