It might be just the price, some of the BM beers are quite strong making them expensive for the area and of course some people just won't drink stronger beers. When I went to the Loughborough one last Autumn the guest was Bateman's XB which seemed to be generally a popular beer up there and was cheaper then the Blue Monkey beers.
There are a significant number of people who go to the William in Leyton and will only drink the Bethnal Green which is Brodie's only boring brown beer.