Because they were smarter than the NIV translators...Do you refer to God in heavens even tho that is a correct phrase or do you refer to God in heaven, which is a correct phrase also???
But actually here's a more complete translation:
shâmayim shâmeh
shaw-mah'-yim, shaw-meh'
The second form being dual of an unused singular; from an unused root meaning to be lofty; the sky (as aloft; the dual perhaps alluding to the visible arch in which the clouds move, as well as to the higher ether where the celestial bodies revolve): - air, X astrologer, heaven (-s).
Apparently the Hebrew word has two forms, The second one being shameh...And as written if the second word is used it can incorporate the part of heaven with the clouds as well as the part of heaven with the celestial bodies...It does not however mention the third part of heaven which is above the massive expanse of water that is between the 2nd and third heavens...
And what probably prompted the KJV authors to use heaven instead of heavens is that in Gen. 1:1 heaven had not been divided yet...It was just one heaven...
Those KJV translators were some smart cookies...
...why not use the original Greek and Hebrew?