It might not be just because of the runway length although it certainly is tight even for the 767. By the way, Bristol's runway is actually a metre narrower at 45m wide rather than 46m at most other UK airports, but I doubt that will be a restriction.
The apron area is quite small and parking is little cramped at most areas, perhaps the wingspan of 197ft on the 787-8 compared to only 156ft 1in on the 767-300ER is the problem??