Because you need about 6" of backspace in order to run a 275 in the front and clear the fenders. You can't run that amount of backspace with 15's, 16's, or even most 17's. For example, the "Bullit" style 17" mustang rims hit the outer tie rod at about 5.6" of backspace, but the Cobra style rims apparently clear, just barely, at 6" of backspace. 18" rims eliminate the issue of outer tie rod end clearance, as the outer tire rod goes inside of the rim space. Which allows even more backspacing, although then the frame starts to become a clearance issue.
In the back it obviously doesn't matter, on a Duster with stock location springs and no modifications to the quarter lip a 275 wide tire is a piece of cake, regardless of whether its on a 14" rim or an 18" rim.
