1) Laziness. People seem to think that the Hebrew form is too long, and thus vibrating it is more difficult. I've used it for years without any difficulty. Just spend slightly less time with each syllable and you'll be fine.
2) Many people seem to dislike that the Hebrew name is similar to that used for the Da'ath sphere: YHVH ELOHIM, and some others actually think that, because the word Da'ath is mentioned here, it should instead be used for Da'ath. This comes from a complete lack of understanding of the Tree of Life and the God-Names attributed to the Sephiroth. YHVH Eloah va-Da'ath is the God-Name of Tiphareth. Full stop.
3) IAO is Gnostic, and is more Greek than Hebrew (although it isn't really any language at all). Using four Hebrew names and one Greek/Barbarous name is inconsistency to the extreme. It imbalances things, puts them off-kilter, and in a ritual designed specifically around the notion of balance, this is completely contradictory.
So why in God's Name (pun intended) do many people still opt for IAO instead of the original Hebrew?