You ask why? That's a lo-ong story. Back in the day, you usually paid for the hardware. If you could afford a computer you could usually afford a staff of programmers to get your software running. A cross-platform operating system (UNIX) didn't even get started until the seventies. If Bill Gates did, too, he was one of the first people ABLE to offer proprietary software, and the Free Software Foundation was founded by old time grad students who didn't like this new innovation -- there were advantages to what could be done with software under the old system where it was a freebie thrown in with your computer and -- ka-blam.
Skip ahead about ten years. To the early nineties. A Finnish grad student has just released the Linux Kernel to run Free Software programs and in South Africa a poor Business Grad Student has just bought a computer. In South Africa at the time you generally ended up buying the OS as an extra, and they were not about to give a price break to Mark Shuttleworth because he was in a business program. A friend who worked in a computer lab gave him some floppies with Slackware, an OS/Distribution with the Linux kernel, GNU Tools, and X-Windows on it, which he installed with the understanding it was free.
What Linux was in those days was exactly a cheap UNIX clone for the PC. People had been using Linux pretty happily for the last twenty years, and most of the internet was (and is) hosted on it. Mark Shuttleworth is a very bright man and found he had few problems keeping up with his coursework while doing it (and teaching himself to program) and when it became time to propose a project he could do his thesis around, he began thinking about doing an internet company. Thawt, Inc. provided those security certificates your web browser is always asking for. It was hosted on some "servers" (old PCs running slackware and Debian) in his parents garage and he got his degree. He also sold Thawt, Inc. to Verisign around the turn of the millenium for almost a billion rand. In his spare time he did some development work for the Debian kernel, and was really happy with what Linux did for him.
This is where things are important. Shuttleworth lives in London now but Ubuntu and the Ubuntu Foundation are South African Distributions. Ubuntu itself is an african word which translates roughly as a Human philosophy, or even humanist if you don't get too caught up in european ideas of humanism. Canonical Software, Ltd. and the Ubuntu Foundation both headquartered in South Africa maintain it, and provide security and make sure that just about everything is safe and reliable. This is truly a GLOBAL operating system which he bankrolls in an attempt to give back to the community which made him his first billion.
The OS itself is very similar to Debian. If you booted into a Debian system it might take you fifteen to twenty minutes to see the difference (and unless you like computer science want to go back to Ubuntu). The biggest differences are ease of use issues, security, and HOW HE SELLS IT. Ubuntu is the only distro which takes a different name for different desktops. More technically oriented ones will offer you your choice of desktops (though Slackware dropped Gnome in 2003) and the KDE version of Fedora, say is called Fedora, as are the Gnome and XFCE versions. Since it makes a difference to people who aren't computer-savvy, Shuttleworth's brain said, "we'll brand them differently". He's an MBA after all. You can BUY support from Canonical. You can also buy this and that which go to support the idea of an operating system for people who can't afford one -- like Mark Shuttleworth back in grad school. Ubuntu is the best-marketed FOSS program out there, but it is supposed to be "Free as in Freedom, not as in beer". You don't HAVE to pay for it, but it's nice if you do.
Oh, one more important point: Ubuntu's people actually code very little and very little of what they code will you see when you turn it on. Every part of the OS and the programs on it is actually provided by a different vendor. Development work goes on irregardless of Ubuntu's success as a distribution. Compiz is available for ANY distribution, not just Ubuntu.