In my case because I use it on the desktop, I like it and I'm used to it. Ubuntu may well be similar to Debian, however I've never used Debian and I know Ubuntu is similar to Ubuntu.
1) Since it makes such a nice desktop, I can run the same thing on desktop and server, which is handy, and a good way to reduce nasty surprises.
2) The regular release schedule is a good way of doing things.
3) I can run it on all the computers in the house, including my wife's, which makes it less of a hassle to keep track of.
In my case because I use it on the desktop, I like it and I'm used to it. Ubuntu may well be similar to Debian, however I've never used Debian and I know Ubuntu is similar to Ubuntu.