It is well known (at least throughout the gaming audience) that the Xbox 360 has the record for the highest attach ratio for games ever. Why? Ads? Target Market? Anything else?

Its audience is incredibly passionate about gaming, and about the type of games that it excels in. Not much more to it, really.

Achievements.

The bundled games count as well so everyone that buys a Pro or Elite has already added two games.

okay, its audience, but how did Microsoft attract such an audience, as opposed to Nintendo or Sony (not saying they didn't get a good audience, this is one of the best gens ever, but MS seems to have gotten a more passionate one)?

noname2200 +1
badgenome +1
rhisc +1

My theory is that games like Halo and Gears attract an older crowd.  The 18-35 year old male that marketers covet.  These are the people that have more money to spend on video games.  This is also their main hobby so they are willing to buy the latest releases thats why 360 games are front loaded.  Like Pacther says they are choosing to spend their money on video games over lattes.

If you make good games, they will sell. The 360 just has more good games. Partly because it's been out a year longer, partly because Microsoft bought some of them, etc...
Metacritic, based on a score 75 or abovE:

360: 212
Wii: 66
PS3: 125

The bigger question... why isn't the 360's attach rate 3 times as high as the Wii's, given the difference in number of games worth attaching...

The way I see it, anyone that is willing to risk/put up with the RRoD issues (yes, I know it's not as bad as it used to be, but it's still a prevalent issue) must have a huge number of games they want for that system.
It's also become pretty crystallized as a hardcore system, which means that gaming is a top priority for its players, hence they will make a point of buying the latest game of whatever their genre of choice is.
I think the XBL community also plays a part in this, as peer pressure will push that hardcore phenomena even further. This is because many games come with online presence, and so when one person gets the latest game, it pushes all their friends to also get that game so they can play together/not fall behind their friend and then the process repeats for all THEIR friends.

Yes, but the "year longer" thing isn't really true. The console's attach rate hit three games in just a few weeks, right away in the console's life. Sure it has boosted the attach rate later on and nowadays, but it doesn't explain the early attach rate numbers.