I think a lot of it is that it’s direction was a bit different. Halo was built on basically immortal Master Chief. Reach had Spartan 3s, which weren’t as “Master Chief”-y, and we see every one of em die on screen as part of the story. That is a pretty stark contrast to Chief never dying. Additionally, if IIRC, the multiplayer was meh, and multiplayer was a huuuuge deal at that time. The story was great (even despite what people said about it at the time) but the multiplayer didn’t carry the game, so it was generally a “play the story, go back to Halo 3” kinda game.

My perspective on it, tho.