You can get me ranting on this point. There are lots of good stuff out there that you will not find unless you broaden your horizon.
But people do not because of the Fandom Condition. Essentially, when you become a fan of something, you remain almost completely devoted to that fandom and things like it. Take this site, for example: a lot of times, people will ask for recommendations of books "like Brandon's".
This has a couple effects. You get an exclusionary culture in the sense that there is now a specific canon of books to be read in order to interact with members of the Fandom. Second, you reinforce the narrowness of your perspective so that you can only understand/like things of the fandom. Anything with a different viewpoint, style or genre becomes difficult to get into. And because we like to reinforce our biases, we claim this is because all this other "stuff" is inferior to the product of our fandom. That's why we end up with sincere, absurd statements like, "Harry Potter IS the best series of books evah!"