When you were learning hiragana , did you think that why「を」is pronounced as 'o' (same as 「お」) instead of 'wo'?
Actually, the old pronounciation of 「を 」was 'wo', but in modern Japanese, it's pronounced as 'o' (without the sound of 'w' infront).
「を」is widely used to mark the object of an action (eg,りんご を 食べます）, so it was retained, sharing the same pronunciation as「お」.
In the past, there were hiragana that were pronounced as 'wi', 'wu', 'we', 'yi' and 'ye', but they were not used because their pronunciation was similar to that of hiragana of 'i', 'u', 'e'. Eventually, they were merged into the 'i', 'u' and 'e' in modern Japanese.