Timing, competition, still lingering bad PR, and I have to say I haven't seen much marketing of it at all. The review scores will save the franchise.
I felt the same about Forza Horizon 2 last year, that only sold about 300k first week as well. Outstanding game, best on X1 IMO, but hey...
PS, PS2, Gameboy Advance, PS3, PSP, PS4, Xbox One