That large box on the right side of the tractor that houses the heat exchanger might be the reason. No doubt it's a tough job designing an attractive and functional cab for a tractor that isn't offered with one, and the heat exchanger has to go sowewhere.
Overall though, I think that Curtis did an amazing job with this one. Some (many) aftermarket, and even some factory cabs look absolutely awful to me, but this is really top notch.
