First. Ubuntu is GNU/Linux operating system. All those who jealous, please hold your peace. Ubuntu pushes forward the same thing that you do. It’s in a user experience, stupid.
Second. Ubuntu is made with usability in mind. All Debian zealots (who “translate” Ubuntu as “can’t install Debian”) are probably correct from technical standpoint. They just forget, that computer is a tool, and I (and many many many) other users do not use Ubuntu, or Debian or Gentoo. They use computer. Surfing the web. Reading email. Watching
porn movies. These sorts of things. So we don’t actually care most of the time what operates our computer tool. That’s why many of us like Ubuntu.
Third. Ubuntu has a commercial support of involved vendor. This in turn means that there will be someone who will push it further while money is in the game. Given that Linux OSes came up greatly up until now without formal support of many distributions (except maybe Red Hat and Novell), another player is definitely a plus, even though Red Hat and Novell do not see it that way.
Forth. Ubuntu is about community. Everyone knows it by now. For such a project to be successful, it has to build a community around it. There was a basis for this community in the first place: Debian. But Debian folk missed the usability part. Ubuntu created a community which is fun to be in. This leads us to –
Fifth. Ubuntu is a successful business model. Take some niche but wanted and/or undeveloped area. Find negative aspects in existing offers. Make what other don’t. Eat them alive.
Sixth. Ubuntu is making it with a good planning and within the wishes of their users. Who said that I will always want open software? Yes, I’d prefer it to other one, but hey, first and foremost I want my tool to work. “No 3d drivers? You mean, not at all? Not even those non open source ones? No, thank you, I will use Ubuntu instead”.
Seventh. Ubuntu is doing it for every platform. Desktop. Server. Mobile. Internet Kiosks. Ubuntu is everywhere.