A big problem with PVC is that it is brittle and will shatter/break. If you used Schedule 80 PVC pipe, it might work. Schedule 80 = thick wall and stronger. Schedule 40 = "typical", thinner wall and weaker.
Steel is ductile and will bend, not break. That's a good characteristic to have when you have a loaded roof rack and hit a big bump in the trail. I could see the PVC roof rack breaking.
I'd also be worried about UV degradation of the PVC, although painting it would probably take care of that concern.