A CO move to BHD prevents the route ever becoming wide-body...and given the good loads that CO are carrying, a wide-body is not inconceivable. Once 787s start joining CO, that will free up 762s and 764s for other routes...and they can't use BHD!
Also, don't be surprised if the any eventual planning approval for the new runway puts some form of limit on aircraft size...and I'd be surprised if that limit allowed anything more than a 738 or A321...
Finally, what is the performance of a fully-loaded 757 off 2300m or thereabouts? Probably sufficient for TATL, but leaves little room for comfort. Throw in the more regular closure of BHD due to weather, and the operational reasons for CO sticking with BFS are probably compelling.