Because Halo 5's campaign is heavily co-op oriented, what with the Fireteam feature and all. Campaign matchmaking would've been a fantastic feature to supplement that, the reason people are making a fuss about Halo 5 not having that particular feature is because Halo 5 in particular is the game that should have it. Combined with the lack of splitscreen, for a co-op oriented campaign, they do make it difficult to find co-op buddies. So I actually agree with the OP, they really should've added campaign matchmaking.
@@VapourKill By the way, Vapour, welcome to the forums! As BATMAN said, this isn't 343i's official site but you should stick around here!