the reason player counts are low in extraction is because THERE ARE 2 MAPS!!!!!!
That's definitely part of it. Part of it (which isn't captured above) is that the population for that mode has never been very high, even at release. It spiked and then dropped fast since most people wanted to play HvV and GA (and now CS). Halo 3 saw the same issue when they started adding a whole bunch of modes and Halo: Reach addressed it by putting said modes into a single playlist. We're currently discussing things like that. Just adding stuff to it won't bolster the player-base enough.