Popular Culture 101, again

One thing I've never been able to understand in telefantasy fandom is the people who become fans of one single programme, and feel that as a result of that, they have to hate all other programmes (particularly similar ones; I've lost count of the number of Doctor Who fans I've met who automatically hate-on Blake's 7, and vice versa, even though they shared many of the same writing and production team and are, as Corpse Marker demonstrates, close enough conceptually to take place in the same fictional universe). I mean, if you have a child, you don't automatically hate all other children; if you get married/civil partnershipped, this doesn't mean you automatically hate everyone else of the opposite/same sex; if your favourite food is hamburgers, you don't automatically hate every food that isn't a hamburger. Also, thinking about it, it doesn't seem to work that way in literary fandom-- people who like, say, William Gibson, don't automatically hate Bruce Sterling, Pat Cadigan, Neal Stevenson et al.; indeed, to judge by the "If you like X you'll also like Y" adverts in bookshops and lit magazines, it's assumed that fans of one author will be actively looking for similar authors to enjoy. So why is it somehow different with television? Answers on the usual postcard please.