Because it works in form factors that an A10 doesn't work in. It presumably has better graphics performance AND better efficiency. I shouldn't have to explain the ramifications of:
1) better efficiency
2) better graphics performance
3) works in ultra portable devices (probably)
To you, do I? You can't use the A10 in anything but a desktop or HTPC. The GT3E Iris pro works in MANY MORE form factors because it has better efficiency, lower TDP.
Now i'm just speculating on a lot of this. But if the GT3E works in ultra portables such as the Macbook air and ultrabooks, AMD has a reason to worry. A big reason to worry. Hell, nvidia has reason to worry as well. Pretty sure apple would drop all nvidia discrete chips, most of their products already are using only HD4000.