It actually fires the 7.62x51 NATO round, same as the G3, FN FAL, M240 series, M-14, and a host of other weapons. AKs fire the 7.62x39.
As for why, one can only speculate that the users wanted a round with a little more punch than 5.56.
- ThreeTracks
From the story:
SCAR-H can be quickly converted to fire AK-47 ammo (the 7.62x39 round) with a changeout of the barrel and receiver.
Other than behind the lines use, this seems less than usefull. Even there, if it does not use AK clips the time consuming task of transfering amo from captured clips to your own clip pretty much makes that a PITA.
You may have scrolled past this item in the story:
"SCAR-H can be quickly converted to fire AK-47 ammo (the 7.62x39 round) with a changeout of the barrel and receiver. "