Because the drivers find the Volvo cabs comfortable and JJs just seem to like using Volvos since euro 6 emissions standards were brought in. Volvos look better on JJs bodies than Iveco strallis.
And one of the drivers informed me that since they got the Volvos a year ago the trucks have been slower than the previous ivecos they had. He said that he had have to work overtime as the arms have been slowed down and these trucks are 2wd.