Here's why:
* "e" in "repetition" is not silent: It's not a silent "e" at the end of a word, which would usually indicate a short vowel sound.
* "e" follows a consonant: The "e" is followed by the consonant "t." This often signifies a long vowel sound.
* "i" is short: The "i" in the first syllable ("rep") is a short vowel sound.
Therefore, the pronunciation of "repetition" is rep-ə- ti-tion.