Here's why:
* Long O sound: This sound is usually spelled with the letter "o" followed by a consonant or a vowel (like "oe" or "ow"). Examples: "go", "boat", "snow".
* Short O sound: This sound is usually spelled with just the letter "o" without any additional letters. Examples: "hot", "dog", "top".
In the word "cone", the "o" makes the short o sound, like in the word "on".