All boutique distros repackage repositories, Mint is just Ubuntu repackaged with the Mint dev team adding their own packages to the mix. If a "beta" package makes it into Ubuntu main, then that same beta package is going to how up in Mint the next time they do a version bump as Mint is downstream of Ubuntu and Ubuntu is downstream of Debian. It's how they can manage an entire distribution at a fraction of the manpower.
The reason for Ubuntu is simple, over 90~95% of all instructions on the web are written for it. When anyone googles "how to install application X on linux" or "how to get game Y working on linux", what shows up is instructions on how to get it to work on Ubuntu. There may or may not be other distros listed, but Ubuntu will definitely be one of the ones listed.
This is much to the dismay of Arch and Gentoo users, not to mention all those minor communities who go tribal over userbase.