> But, for whatever reason, Mint shows up at the top of the list
Seems rather obvious to me.
I assume most Ubuntu users are not Linux geeks. They have no idea that there is "more to Linux" than just Ubuntu (and they don't care.) So only the disgruntled Ubuntu users go to DistroWatch. Once they are there, they don't click on Ubuntu because they've used it. Mint gets clicked on a lot because it is the most-discussed alternative to Ubuntu.
It's sort of like wondering "If 95% of desktops run Microsoft Windows, why isn't the Microsoft home page the most popular page on the planet?"