Because it's the part that they had and felt they could use to achieve such a quick shutter time?
Or because Verizon, knowing the GNex to be the superior device in their arsenal of Android devices, intentionally sabotaged the camera to push users to the Rezound?
Or because the software hasn't been tweaked to perfection?
Or because Samsung really wants people to buy a GSII.
Or because Samsung was paid by HTC to make the camera poor to push users to the Rezound instead of the Galaxy Nexus.
I think I've covered all the bases with these ideas; from utterly absurd (but hey, some really dig that stuff), to somewhat likely. ;P
Anyways, can we just be honest with ourselves? Nobody here has any idea; for all we know, this was the right choice by far. Or it was a stupid one, based on maximizing profits. We don't know, methinks.