West Brit,
It's a tad futile a conversation to say that for the same routes ex BFS there are higher numbers. If easyJet ops were into BHD and Flybe into BFS the trend would swiftly be reversed. Equally, BHD LGW was busier than BFS LGW for a time when capacity and low fares were more widely available. So price does play a big part.
Aer Lingus equally have reduced aircraft size and numbers have declined proportionately in turn. The level of passengers on BHD LGW are as good as BFS in proportion to the seats available. If airlines don't put the capacity into place, the passengers won't fly.
There are a cocktail of factors at work here, yes price. But frequency, number of seats, schedule convenience, relevant strength of the airline to compete, how established a route is etc.
Just my perspective on the subject. While this post will defend the performance of BHD, it could always be argued that the proximity of BFS as too far out of town is a nonsense, bourne out by the figures....