I think it's a bit of both, but I think Guinness drinkers are very hard to get to move onto something else. And they are used to it being a little bit cheaper than everything else, which doesn't help. Many's the barman has commented to me about how Guinness drinkers always know how much their pint cost.
Also I think a huge part of it, and unique to stout almost, is draught in cans. Very hard to get people to shift away from the draught.