It's worth reading the accepted answer to this question, but to specifically answer you:
「ヲ」 is not really "wo", at least in the sense you expect it to be. It's the same as 「を」, which is typically pronounced "o" and used as a particle to mark the object of a clause. It is sometimes, depending on the dialect or even just the individual, pronounced as "wo", but you'll find that the "w" isn't necessarily a strong sound - just like how "ra" is only an approximate pronunciation for 「ら」, or "fu" for 「ふ」, if you try to pronounce "wo" like Keanu Reeves says "woah" then you're doing something wrong.
So using 「ワォ」 for "wo" helps get across a little more of that meaning, and for the transliteration of foreign words you will exclusively see this rather than 「ヲ」 (which, outside of its particle use, mostly only shows up in names like 「カヲル」 for semi-historical reasons).