I think you are seeing a reaction to the splintering of Linux. Most people don't want to research which variation on Linux meets their particular needs. They are content to go with the first distribution they find that seems to meet most people's needs. At the moment that is Ubuntu. It's easy to find, it's easy to use, it's website is easy to navigate, and it's an easy name to remember. The fact that they are expanding into alternate interfaces (Kubuntu) and devices (Ubuntu Netbook Remix, Ubuntu MID Edition) just makes the decision easier.

Texrat's analogy is a good one. In the early days of PCs most users never knew there was any DOS besides MS-DOS. Eventually it became not just the de facto leader but the only DOS most people would use.