You just don't mess with success.
It's not at all uncommon for automakers to rename/rebrand various models. Sometimes it's necessary for many reasons. Other times it could turn out to be the worst idea. Fortunately Lincoln was wise enough not to mess with a good thing. Instead of rebranding its full-size SUV with the MK nomenclature, as was done with the rest of the lineup, the Navigator name will remain now and in the future. Cadillac is also following this exact model with the Escalade, avoiding yet another alphanumeric naming strategy.
So why didn't either brand change their SUV's names to match the rest of the lineup? Because everyone is familiar with them. Changing their names would have confused some customers and each SUV has a loyal following. Surprisingly, as Lincoln found out through some market research, even Chinese buyers are familiar with the Navigator name. Lincoln sells about 8,000 Navigators per year in the US, and it's actually the most profitable vehicle in the lineup. You don't mess with something that ain't broken. The Navigator was given a refresh for 2015, but a completely redesigned version is supposedly in the works. And yes, it'll be called the Navigator, too.