http://www.jubling.com/eight-reasons-why-ubuntu-is%20the-linux-poster-child.html1) Its large user base. Because Ubuntu is so popular, it becomes even more popular. This viral marketing was the same thing that made YouTube successful.
2) Hardware support. One of the most common complaints among new Linux users is that their new hardware is not supported by any Linux distribution. Well, with Ubuntu, hardware support is added within days of the hardware coming out.
3) Its ease of use. Lets face it, you can't make a mainstream distribution without ease of use. If you tell a Linux beginner that they will have to type out all their commands into a console, they will have a Windows CD in their disk drive faster then you can say "Open Source".