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
prejudicepreference. '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'.