I'm with you. Rust. Ethanol oxidizes to acid when water and air come into contact the gasoline, and unlined steel accelerates the process. An aluminum tank would be more resistant, and it would be lighter than a steel tank, lowering the center of gravity a small amount. It will also reflect a certain amount of radiated heat, keeping it cooler.
Of course, you could just line a steel tank with a polymer, but that may be too pricey for Power-Trac (liner cost, labor). Small volume manufacturing has a constant tension between fixed and variable costs as production numbers varies. Add in not wanting to switch manufacturers very often to be able to stock spares, and I think PT has real challenges.
I also suspect that their parent/sister company, CLA, is having real challenges with the shrinking local mining industry, which lowers the purchasing volumes.
Still, I would appreciate an aluminum tank. It has "looks".
All the best,
Peter