Because a single 6870 would be a downgrade according to this thread.http://www.tomshardware.com/forum/304006-33-5770-crossfire-single-6870
Maybe if your situtation was "should I get a SECOND 5770 (you only had one)" or get a 6870, I'd say get a 6870. But you already have two 5770s, so yeah.
According to this CF 5770 is about equal to 5850 in performance.http://www.guru3d.com/article/radeon-hd-5770-review-test/22
and according to this the 5850 is about equal to the 6870.http://www.tomshardware.com/reviews/radeon-hd-6870-radeon-hd-6850-barts,2776-10.html
So yeah, it would be an upgrade in some titles. But not a worthy one.
EDIT - Ok now I see your changed your sig to say you have CF 6770s? Well I don't know what you're asking then, lol.