I LOVE Halo 1, I liked Halo 2, I HATE Halo 3.
Halo 1 was good. It had a good mystery-SciFi story mix. Fun times, good action, interesting people. It attracted a large fan base. Understandable.
Halo 2 was a forced sequel in my book. It was good, but didn't live up to the hype. Better mechanics and graphics made it worth the buck. Fan base grew.
Halo 3. It did NOTHING. Slightly improved shaders, did they even add a new gun? The story sucked balls, the voice actors changed, and the ONLY thing that made it ANY good is the Forge and the multiplayer. When you present these arguments to half of that built up fan base, they say (quoting my room mate) "Y3AH, BUT THTA MULT!PL@YER IS TEH B3ST EBAR!!1!" (all punctuation used for accuracy).
According to them, having a good multiplayer component excuses the sins against story and gaming that comprise the rest of the 'epic conclusion' to the trilogy, which I think they could have rocked with if they ignored the multiplayer a bit (which now has nothing but drooling 8 year old trolls anyway).
So, in short, it's because the 3rd game was crap, and the fans defend it as if it was a bible written by God himself. Compounded annoyance; I see a blue sky but the man next to me INSISTS it is red and will not change his mind. Additional games, like Halo Wars and the 2 new ones announced just prove they are milking the series and I have no faith in their quality since I think there wasn't enough interesting mystery or other elements that defined the first one left. It will be the same shooter as Halo 3, with no surprises. Why would you be happy for that?