It's because the sand is at 600c which would wreak havoc if you had water pumping through the tubing. Air is cheaper to do handle at those temperatures.
Might be a crazy idea but maybe they can just use smaller tractors. I'm not sure if we have the technology to build Smaller tractors. But since they are needed maybe there could be a Lot of R&D to make a tractor that fits under the space available in these installations.
It's because the sand is at 600c which would wreak havoc if you had water pumping through the tubing. Air is cheaper to do handle at those temperatures.