They made a decision to promote it as Carsland. Not too often have they changed the name of a new attraction, let alone an entire land, once they have made it known to the public.
i was thinking the same thing and then i realized, Disney has renamed an land before (more than once to be exact). Mickey's Toontown Fair at MK in WDW originally OPENED (not planned) as Mickey's Birthdayland, then later changed to Mickey's Starland, and then once again to its existing title. These name changes also revolved around renovations and additions to the land, but still the name of a whole land has changed.
But back on track to the Cars Land topic, I agree that the name should be changed to Radiator Springs, Route 66, Cars Country, Cars Canyon, something of that sort. if they really want CARS in the name, thats fine just not Cars Land.