Why not say 'sci fi'?

I used to be part of science fiction fandom, in my teens and twenties, and one of the things that drove us crazy was when people referred to sci fi (pronounced sci fye) instead of SF. I think it was mostly a matter of defining boundaries - we the real science fiction fans knew the correct word to use, while the mundanes (our name for non-SF fans) didn't know what real science fiction was and called it sci fi. Calling it sci fi was a way to belittle the literature (and by extension, us).

Normblog provides a more elegant and literary reason:

Readers, I'm here to tell you that I've now thought of an argument in favour of my prejudice preference. 'Sci fi' is supposed to abbreviate 'science fiction', but it is spoken as if it rhymes with 'hi fi'. What kind of sense does that make? If I say 'in the circs', I wouldn't pronounce 'circs' to rhyme with, say, 'larks', so that it came out 'sarks'. If I say 'peeps', I don't rhyme it with 'hopes' and call them 'popes'. And so on, you get the picture. Accordingly, 'sci fi' ought to be said as if the second syllable was the beginning of the word 'fiction'. But no one says it like that. It would sound silly, as if it had been interrupted by a sponge suddenly being thrust into the mouth of the speaker. From now on I'll be urging this silly pronunciation upon all who say 'sci fye', in the hope of shaming them towards the more elegant 'SF'.