Why? Halo has and always been a multiplayer focused game with the campaign as a second priority. Of the seven friends I know who played Halo 3 regularly only 2 of us had finished the campaign and one of them was because the two of us wanted to play through the co-op campaign.
U wot m8?
Halo 1 didn't even had multy, it was a scrappy (yet amazing) LAN party all the time.
Halo 2 had 15 missions of Campaign, and an awesome multy, but wasn't multiplayer-focused (as Spectre said, it was some sort of add-on).
Halo 3 focused entirely on Campaign, with muliplayer maps being added by 343i.
I can't really say anything about Halo Wars, Ensemble being disbanded even before its release (MAY GOD DAMN YOU MICROSOFT)
I wouldn't say ODST had a multy, although I loved Firefight and I still hope we will see it make a return in H5
I loved Reach's campaign, but its multy was unbearable. Even after 343i took over Bungie's control of the game, it was unplayable. I think Bungie didn't even care about making a decent multy that time.
And for H4, yes, I agree, alas, saying that it was multiplayer-focused. And it has a friggin' amazing multy. But the worst campaign in the serie. Maybe is it just me hoping that Halo hasn't always been focused on multy? Maybe.