It's not just the shift in time period from the rest of Frontierland, it's the whole conceit of Cars as a concept.
Cars Land at DCA works because it's a self contained land, its own thing separate from the rest of the park.
Frontierland is about humans, animals and their history, narratives and relation to real world settings. Cars is a universe where everything, even little bugs, are vehicles. There are no humans in Cars and nothing is human scaled. The Country Bears may be talking, anthropomorphic animals, but they don't live in a universe where everything is a bear. The New Orleans of Tiana's Bayou doesn't have living cars, the dinosaur bones on BTMRR are not the skeletons of Cars etc. You could have an off road trip through Nature's Wonderland that carries the established time progression of FL/LS, but because every speck of the parks has to be IP, we're stuck tying this to a Pixar franchise that aesthetically and conceptually doesn't fit the location.
Then there's the simple fact that the vehicles have big cartoon eyes on them, and just look silly next to the rest of Frontierland and Liberty Square.