People will keep buying Cadbury because it's ubiquitous - every corner shop and station bookstall has it on display. You have to go out of your way to buy the real thing and that's a real shame because proper chocolate is so much nicer.
That's very true. I often think the same thing with energy or broadband suppliers. You could be the absolute best company ever for people to use - lowest prices, best product, best customer service, most reliable etc - but the vast majority of people will still instinctively go with one of the very few big companies who advertise incessantly, all claiming to be the best, but actually invariably delivering the worst service overall.
They rely on most people not really knowing that alternatives exist or, if they do, how to go about finding them - and thinking that they're probably aimed at hipsters with more money than sense anyway.
Taking broadband for example, if you look at ISP Review, they have a list of the top-10 rated providers. With the possible exception of Zen, at number 7, not one of them is well-known, much less a household name. They used to do a top 50 (maybe they still do - they've changed the site around!) and none of the big well-known ones featured in the top 30 for customer satisfaction.
Why this attitude translates to chocolate, I've no idea - there's no contract, no signing up, no switching - just a small amount of money for a one-off purchase - but I think all the years of advertising have ingrained themselves in the common psyche, as well as the fact that you get a lot more for your money than you do of the decent stuff.