Having sold UK's for a couple years, I think I have a pretty good handle on this one. I probably put several hundred guys into their first kilts.

Part of it is price. The Original is less than half the price of the Workmans or Survival. It's not that much cheaper than a Mocker, but the mocker has a much 'skirtier' looks that doesn't bode well for a guy worried about how it will be perceived.

Black is a great color too. There isn't any risk of it being seen as feminine as a khaki kilt might be, and it goes with more stuff than the olive kilts or camo. Black is also the mandated color for Rock, Punk, Goth, Biker, or most of the other 'tough' guy groups.

In short, the black UK is a gatekeeper (more commonly known as 'gateway' here) because it screams "THIS IS NOT A SKIRT! IT'S A KILT AND I AM A TOUGH GUY!" and it does it at a decent price.
It's a great way for a guy to test the kilted waters and still have a salve for a risk averse ego.

(Not to be taken as a slam for guys who started on black kilts. It's perfectly reasonable when taking such a huge step outside the 'norm' to hedge your bets.)