Here's why:
* The "o" in "compose" is a short vowel sound. It's pronounced like the "o" in "lot" or "hot".
* Long vowel sounds are typically associated with a single vowel letter followed by a silent "e" at the end of a word. For example, "hope", "mate", "kite".
Therefore, the "o" in "compose" has a short vowel sound, not a long one.