Come on man, there are pretty easy to spot reasons why Reach is the most populated Halo game out there at the moment. The better statistic to look at is Halo games populations 1.5 years after launch, Halo 3 was doing a lot better than Reach is at this point in its lifespan.
I mean I can only speak for myself and the small group of people I know who really play Halo, but except for HaloGaf, no one I knows still plays halo, some people play Halo 3 here and there, but ultimately most have given up on the franchise. Halo 3 is still a great game sure, and thats why people still infrequently play it, but considering the timespan, and the state the game has been left in, rather than move on to Reach, or play a gimped version of Halo 3, most people I know just dont bother anymore. Mostly probably because its now literally impossible to go in with a full team in the playlists they play most and get a game. Im kind of tempted to say im part of that statistic, except I still probably venture into Reach more than most I know do, I only end up playing a few games here and there though.