A very stupid one. Those armies need a complete re-write, and in the case of Dark Eldar (and possibly SoB and DH), a complete new plastic range. Despite popular internet belief, this takes years and years to complete, not a weekend with a few cans of beer. It's already been revealed by Harry on Warseer that Dark Eldar, Daemonhunters and Necrons are all being worked on at the moment, and have not been forgotten about. Big projects take time, deal with it.
Or would you rather then slap together a half-assed codex and mini's so more could be released sooner?