Because it will still be circulating in other non-Island communities (UK mostly) and because there will be some on the Island who will either not take to the vaccine, ie., their body will not accept or reject the vaccine (remember that none of the jabs claim 100% efficacy and some who simply refuse the vaccine.
I agree we need may need to allow it in but only once we have reached the level for heard immunity to develop. At the moment the experts don't know what that % will be due to the quicker transmissibility of new variants.
Based on what I've read it will require monitoring for at least another 2 years I reckon.