Also because the UFC expects to get buys on brand name alone, in addition to the fighters. In boxing you're paying for the main event, everything else is secondary. Dana would want you to believe that you're paying for the full card with the main event as the attraction. It's worked to an extent.
It's also the way it is because the Klitschko's dominated from 2004-2015 or so. So no American could really make any noise, but in addition, Wlad primarily only fought in Europe. So, watching things on a delay or early afternoon wasn't the best way to market the division to the US audience.