Thing is, it isn't a continuation of the Halo storyline with master chief and everything. Halo 3 was so huge because it was finishing up this epic story we've been following from the start, and it was supposed to have a multiplayer better than Halo 2's. We got a great, somewhat bittersweet ending, but the multiplayer devolved into overpowered melee, grenade spam, and AR spraying.
Following Halo 3, we got Halo Wars which was meh, and Halo: ODST which was good, but could have been DLC for it's 5-hour campaign. Because of those games (and that fact that it wasn't Halo 4) kind of stopped people from thinking of Reach as being THE next Halo game. So I guess it suffered from being a prequel.
My group of friends faced another problem in that we're all starting college now. In 2007, we were just care-free high school sophomores. Now we don't have nearly as much free time to put in hours on multiplayer and some have even drifted from gaming altogether. I would say less than half of the people I played Halo 3 with even bought Halo Reach.
I personally like the game, and think it's a big improvement over 3, but I have a weird problem with multiplayer now. During Halo 3's prime, I could just pop in the game and play for hours on end with or without friends. I got really good, but some time after the Legendary and before the mythic map packs, I just got stale with the game and could only play it with friends. I was so stale, I didn't even play any of the maps released with ODST. All I remember doing before Reach was playing the Halo 2 send-off in April, and enjoying every minute of it because it was such a great game as well as being sad that I had played it more before it's shutdown. So yea.....If I had more time and more friends that bought Reach, I would be playing it a lot more, but sadly that isn't the case.