Brand loyalty, yes, but it stems from the fact that the average pub has the same few beers/brands.
For every time I've had that conversation with someone I've also had a conversation along the lines of 'I went there before, They didn't have "X" but I tried their own 'version', and it was lovely'. Trouble is, I have this conversation with people, while sitting in my local, back with the usual limited choice.
Coming from outside of the two biggest cities, going to the likes of the Porterhouse or the Well is something of a novelty.