It's not unlike the main truck brands. Everyone drives, a Ford, Chevy, or a Dodge. If you want something with a little better quality, like a Tundra, or even the new Titan, you have to go elsewhere, and pay a little more for the same options.
Trek is an American brand, and everyone seems to like them. Most people outside the cycling community who are just getting into cycling, have a tendency to veer towards Trek, Specialized, and Giant. This is not uncommon.
You can thank Lance.