I'll break this down for you, for those of which can't decide about which is better. Here is the detailed explanation of why there is more voters for CoD than Halo.
Call of Duty, first and for most is a shooter just like Halo, but even though they are both FPS type games, they can't be compared, they're gameplay, story line and features are completely different. Yes there two FPS games but you can't say that Halo is not realistic enough and that CoD is not very adventurous. Halo has a very big and complex story line that some can't follow because it's quite difficult and long.
Call of Duty on the other hand simply replays history, until Modern Warfare of course, but by this time, it means nothing as MW2 was a complete let down and the franchise is split by two very different developers. Treyarch can't seem to realise that CoD world war 2 games have had it's day, well world war 2 games have had it's day I should say, not just in the CoD franchise. Treyarch simply keep ruining Infinity Ward's reputation with badly glitched game modes and menus etc. Call of Duty as a whole cannot be measured against Halo.
Halo is set in a long period of time, following a war against an army of races together called "The Covenant" and follows the journey of mankind fighting them, whereas CoD takes place in various different locations and eras the CoD developers wish to take us to. Call of Duty is not as good as Halo, as CoD unlike Halo fails to produce any meaning full story line for it's audience, no meaningful multiplayer, and also no solid purpose. TA and IW simply keep creating for money. Admittedly Bungie may do for their Halo however you can't deny the fact that the specifically design 'Party Games' such as Capture the Flag, Oddball and King of the Hill are great fun with no serious atmosphere needed.
Many people will say CoD is better, but may I ask, have you played through Halo 1, 2 and 3 and also seen at least videos of Halo Reach? To those that vote Halo, we've seen that the only thing your bothered about is rankings and looking good in front of friends when the better and more laid back game is Halo. If you haven't played one of the games, I advise you do research on the one you haven't played, it's a pet hate that I have when people say they don't like something when they've never even heard or seen anything about the particular given subject.
Halo beats Call of Duty, all of them, even MW2. The Reach Beta also beat MW2 hands down, MW2 may be fun but it will never be as good as the Halo universe.