The answer is pretty obvious. Halo was the King of Online gaming on consoles before Call of Duty came along, Call of Duty dethroned halo and cannibalized a large segment of not only its player base but virtually every other online title out there.
It was regarded as the best shooter of its generation for a large time and now Call of Duty is regarded in that manner by many its kind of like Apollo Creed Vs Rocky.
"Core" Gamers want to see call of duty dethroned and many hold the ideal that if any title could do it it would be Halo because of its history)
Its likely not going to happen until some large cross platform title comes along that does CoD better than CoD and then we are going to see the situation mirrored with whatever Game X is, Battlefield stands a very good chance but the length of a game and the lack of screen candy player rewards hinder it to some degree.