I'm a software engineer..
1. I have researched. They use high density FPGAs cause they cost less to manufacture and they fit more capacity. There isn't anything else to it as even most of them explain where they talk about ;'phasing out' low-density..
2. You don't need technical expertise for this. The issue is discussed on every hardware site there is from PC Magazine to overclocking forums.
3. The fact that most of AMDs K8 line don't support high-density modules makes this significant.