I said it already, but I will say it again.
Having a delayed, non-satisfsctory and slow grind included can make something fun (playing halo) feel like a chore and create a negative feedback.
This is a serious problem that affect most of the people. Yeah, you might just play for fun and ignore the unlocks, but thats not a common user scenario. Most of the people see that their time spent in the game is rewarded with something frustrating and slow, so the "prize" per match feels more like a punishment.
They implemented common videogamey stuff without realizing what makes it fun, and that made it not fun. Which is basically 343 in a nutshell.