Because the 5770 is old card with limited capabilities. With that cpu, you would definitely have a bottleneck with your graphics card. 2 5770's would lose to a single 5830 or a Gtx 460.
That has to be the most asinine thing I have ever read.
Two 5770's in crossfire beat a 5850 the majority of the time and sometimes even challenge a 5870.
DO NOT, i repeat DO NOT get a 5830, its huge, hot, and power hungry.
On the other hand a 5770 is a great card that is very cool, small, and efficient, plus it scales great in crossfire.
Don't let this fool put lies into your head.