My logic is that spending around two hours playing on legendary and only getting 4750Xp is not worth the time. I see no reason to do them because spending that time in War games gives you tens of thousands more xp. Reach gave you commendations to earn and some cr for doing it. I enjoyed getting commendations in that game, it gave me a warm fuzzy feeling, the doctor later told me it was just gas. I digress. If you added a better incentive to doing them people might join.
One idea a friend and I had was do all the challenges one month and you earn 20 games of double xp. That would drive your population sky high!