Steel hand trucks have been at work by the millions for a very long time. If you look around a major commercial warehouse or in the back room of a mom and pop shop your very likely to find one or more of these handy tools. They ride around in the back of delivery vehicles and are rented daily by home owners with "stuff to move."
If you were to list the attributes of a good hand truck what would your priorities be? Near the top would have to be strength, enough to do the job plus provide a safety. Hand carts built from steel have it. Steel has served man for centuries and we know more about making it and refining its characteristics than any other metal. Steel can stand up to the abuse and get the job done.
Durability would be a major consideration, particularly under heavy use or a rugged environment. Steel lasts. There are steel hand trucks out there have worn out many sets of tires. The maintenance is minimal with a little steel wool and paint to keep the finish fresh. The right steel and a good design will take the flex and the bumps in stride and most will survive that occasional fall from the loading dock.
We cannot forget cost. They are the economical tool for getting things moved. Aluminum has a substantial cost increase for a given capacity. And given the low purchase price, you can afford to get more than one if you need it. Larger operations may need more than one to cover all of that ground.