Because a lot of the people till have raging boners about Halo 2, which is far from what Halo: Reach is.
My friend who loves the Halo series a lot, still likes Halo 2 the most, and doesn't like Reach that much at all. I am the reverse, I love Reach the most, the only multiplayer I have actually tried and gotten a LOT of progress in.
For me, Halo goes:
Reach > 3 > 1 > ODST > 2 > Wars.
3 will probably be the most anticipated and hyped up game until Diablo 3 comes out, and it was hella fun.
1 started it all, and has one of the only campaigns I go back and play for shits and giggles, because the campaign is actually really fun.
ODST is a Halo game without Master Chief, and it is fucking awesome. The atmosphere, the story, the "I am not an invincible Spartan, but still pretty badass", and the introduction of Firefight, are all really awesome.
2 has a really irritating campaign, and the flood levels are hard as fuck. The multiplayer had some of the most dedicated and competitive fans of any gaming series ever, and people I know that like Halo, still miss it.
Wars is a failed attempt to make a Halo RTS. The story is alright, but the balancing is off, the missions easy to exploit, and the controls are just really irritating to use. If it came out on computer, it would have done a lot better.