Because they have no reason to. Seriously. Linux offers so little of value over Windows 7 to the average user that would justify dealing with a significant UI change and loss of support. The average user has had a hard enough time moving from XP to 7 (and these are honestly almost identical except for a couple of eye candy tweaks). The differences going over to any of the current Linux distros would be too much.
The argument that Linux could ever take off on the desktop is based on the idea that it's equally easy to use for a complete novice. Unless you regularly deal with people who are genuinely novices, you really don't understand just how much changing things scares them. The support infrastructure for Linux on the desktop is just plain awful compared to that of Windows--OEMs do not support it except under specific circumstances, and the average neighborhood computer geek is far less likely to know how to fix an issue with Linux than with Windows.
Converting to MacOS X happens because of marketing hype and chic factor, but at least there is a decent support infrastructure in place from Apple, and the platform is consistent and stable enough that most issues that arise can be fixed.
Windows "just works" on almost any hardware. MacOS X "just works" on Apple systems. Linux does not, unless you are willing to do some digging.