Why would being slow make the Necrons "impossible". There are plenty of FPS games that have different speeds for different classes. TF2 for example.
Because we don't see Tau alot in Warhammer 40K computer games. We saw them in Dawn of War 1 in recent memory, thats about it. Necrons should be a PvE race similar to Tyranid as noted, and Dark Eldar is just half naked Eldar anyhow IMO.
Wot? No Guard! Tau then! 'cause they are different from the WFB races, unlike new Toombcrons and to a lesser extent Dark Kinky Space Elfs. By New I mean Codex new.
We haven't seen Necrons or Dark Eldar either. Tau actually have their own video game. This I still wonder. They would have completely different classes (and just 4 would do unlike with eldar), their differences would be huge if we compare to other factions that are similar to each other (that are already in the game, btw), different arsenal and easily different gameplay. For Necrons, they have more potential than 'nids for PC-race, but it is still a tough call on it.
A Chaos Space Marine says that Dark Eldar are boring because of Craftworld Eldar being in the game. Say, do you see the irony in that? But why?
I'm not saying I wouldn't want Dark Eldar in the game. I can appriciate a Dark Eldar Wych as much as the next Marine, but I am saying that we should go with Tau first as a community since they do not get alot of stage time in the grander scale of things, and most of their army is all about long range firepower.
While I don't want to keep Tau away (and, honestly, I think they will be the next addition no matter what we say here), they do not have less stage time than Dark Eldar or Necrons in 40k. Fire Warrior was even the first 40k FPS, wasn't it?
Necrons, as they have so many easy class options. Deathmark, Lychguard, Immortal, Cryptek, Praetorian...etc.
None of the above, I'd prefer to see IG implemented, they existed in lore and on the TT long before any of the above factions were even conceived.