Isn't Halo2 coming for holidays?
If so, they must've just prioritized the release schedules of important games. Coming at the same time as Halo2 would certainly hurt the D3 sales.
On the other hand, if they release in 2005, Doom 3 will have lost the initial hype that can carry the sales of even mediocre and poor games to surprising heights.
For best sales, they should've released simultaneously with the PC version.