Well, there's a few reasons:
* As mentioned, Google Analytics doesn't have the very advanced features of an Omniture (SiteCat, Discover, Insight, Genesis, etc.). It isn't clear that Google Analytics can't have these features, they just choose not to.
* A lot of people don't trust Google with their data. If you dig into the Terms & Conditions for Google Analytics, some companies' legal departments might get very spooked.
* The phrase "you get what you pay for" is often very well believed in the Business world. To have an analytics solution for free, when folks like Omniture, Coremetrics and others are charging 5 or even 6 figures doesn't seem to make sense.