Here's why:
* The "o" is followed by a consonant ("c"). Short vowels are typically followed by consonants in a word.
* The "o" has a short, clipped sound. It's not a drawn-out, long vowel sound like "o" in "go" or "hope".
Think of the short "o" sound in words like "hot", "stop", or "box". The "o" in "focus" has a similar sound.