I'm sure you must mean 11.10 since Unity isn't used in 10.10.
For the same reason that Gedit is faster than LibreOffice. Like Ubuntu and Debian, Mint has many different desktops. The LXDE desktop, for instance, is specially designed to consume as little resources as possible, at the cost of features. The more features you have, the more resources the application requires. All the desktops that Mint and Debian provides, are also available in Ubuntu. If you're comparing Unity to LXDE, then you might as well compare Gedit to LibreOffice. For some use cases, they're just as good, and you'll prefer the faster one. In other cases, you'll want to make the computer work harder in order to improve your own productivity.