From the Gears Beta thread, lol
I love how people always jump over Reach, so they can blame 343 for the downfall.
This discussion was held long ago, COD4 cam out and with it Halo's console fps playerbase split, COD was a much more approachable and easier shooter to play, and since it was Multiplat not only Xbox peeps were talking about it, so the reputation rose way faster. Reach then changed Halo up, to get those people back, but completely forgot to make the game to also maintain the ones that stayed and so even more left.
This trend continued, till now where the Xbox One isn't even close to competing with the Ps4 and to be fair Halo 5 has not the population decline Halo 4 has.
Also a classic halo experience was released.. the MCC, forget server problems and everything, we can just see how big the interest for classic Halo MP is at the launch sale data( people couldn't know the game didn't work)
The sales were really really low for a Halo title.