Because everything works out of the box with Mint, and I get a beautifully themed desktop along with it. It would take me several hours to do what Mint gives me by default.
It is not entirely true.
My favorite manager has been XFCE, however a few programs that I use doesn't work well under XFCE, hence I'm default back into Gnome.
Mint 9 had just as much problem as Ubuntu, and help from Mint community is weak compare to Ubuntu. 9 out of 10 times that I needed help on Mint 8-10 has been resolved by searching Ubuntu for the solution.
And I love that Ubuntu doesn't insert crappy search on top of browsers, and many programs are Deb package for Ubuntu long long before it is available in Mint repository.
Ubuntu repository is also added in as default by me into my sources.list from Mint 8 to 10.