I have to disargee on two things here.
1. Many people do play Halo Wars. 5000 odd players online at once and 30,000 in 2 hours is many people. If your going to compare numbers with the two most played games, not only on xbox, but in the world across all platforms. The yes, Halo Wars does not have as many as them
2. Halo Wars does not have as many players as Halo and the CoD series, but not for the reasons you listed. Halo Wars is a strategy game, which instantly turns away many players. The remaining players then have a rather steep learning curve to tackle, and being a lazy race of people us gamers are, we turn away now. The people still playing, who are trying to learn, get no immediate positive feed back. You could spend hours learning, and still not win a game online, which is different to Halo 3. You can go into Halo 3 for the first time, having never played a shooter, and after mere games, you will be getting kills, and your team will be winning games. The lack of positive reinforcement for learning turns players away. Now, the remaining few that want to play strategy games, that want to learn, and want to press on though the learning curve come across the biggest killer of all; the rushing! Rushing is imediately frowned upon by begginers because they see it as the easy option, the lack of skill instant win strategy. Begginners do not know how to beat a rush, and can't see any possible way to beat it, this turns away a good 60% of players new to RTS. The ones that remain must now try and learn to beat this rush, and again, its super tough to them, it's has a steep learning curve, and they see no progress at all. They will fail to beat every rush until they finally learn how to, which can take many many games. This again turns away alot of players.
The few that want to think in games, over come the steep learning curve, fight on even though they get no feed back, want to counter a rush, understand RTS means real time strategy meaning to strategise real time, the few players that then spend alot of time, and take alot of losses to overcome the rush and finally get acceptable at the game, and can enjoy the competition are what make up HW's population. So there is bound to be few.
After all that, HW is an extremely simple RTS game, containing only the bare fundementals. So imagine what its like to get new time RTS players into something complex like Star Craft, Age of Empires and Command and Conquer? Theres barely 1000 players online at peak times on the hot RTS of the month.
RTS players are few in number when compared to shooters, so lets not compare them. HW has a huge population by RTS standards, and I hope many players now consider moving up to the next level and getting something more complex for PC. Or, a game company takes the mile stone Ensamble have set, and create a new and better RTS for console.
Arbiter is 1337!