I can't speak for the Cadiz turn specifically, but a lot of lines that have weight restrictions require four-axle-only power, because they generally weigh less.

But... doesn't the AZCA have some six-axle power? If so, then what's going on? The interchange tracks on the AZCA side can handle six axles, but the tracks on the BNSF side can't? That would be rather surprising.

Or, the issue might be curve radius on the wye at Cadiz. If I understand the way that operation normally works, the BNSF moves their cars onto the wye, and then takes the cars that the AZCA left off of the other wye track. (This is two tracks on the wye tail, not the two "legs" of the wye.) But for that to be the issue, one of the wye legs would have to be curved enough that you don't want to use (say) an SD truck on it. That would be a really severe curve; I'd be surprised if that was the explanation.