Tha Robbertster
Member
Because they weren't expecting it.
People were expecting Halo 2 to be the Chief fighting the Covenant on Earth.
It was mis-managed, people felt the game was a bait & switch so that blowback blew on to the Arbiter too.
Also, the introductory Arbiter scene, in the mausoleum is (awesome,) very long and the impression I get is that a lot of people non-interested in the Universe other than the Chief, didn't really care, so allowed themselves no connection to the character.
That & people generally prefer to be Human characters. Us vs them mentality.
I didn't dislike the Arbiter as a character, I actually preferred him over the chief at that time. But I really disliked the levels you played as the Arbiter. I hated the first four Arbiter levels, mostly because you were fighting the flood most of the time and the levels were really dark.
I think that's why the general public disliked the Arbiter. They associated him with the dark flood levels which the general public doesn't like either (I think).