Aluminum alloys, selected properly and with a decent design make stronger lighter assemblies than you can get with cast iron. For equal strength assemblies, the aluminum alloy will be much lighter but will cost significantly more.
With a proper design there are few heat problems with cast iron blocks and various structural members. Heavily worked diesels have been made of cast iron quite successfully for a while now. I think the engineering is pretty well understood. LIkewise tractors, dozers, earth movers etc.
Irrespective of any 4x4 vs 4x2 arguments, most tractors used beyond lawn mower or ego massager duty for agricultural posers NEED to be heavy to git 'er done. Weight means traction and ET (elapsed time) in a test of acceleration is not a central concern in tractor usage or design. If you need weight why build from aluminum and then add weight? You pay a double whammy cost penalty.
There is lots of talk about filling the rear tires, adding wheel weights in back and adding front weights. Making parts out of appropriate aluminum alloys strong enough for the job costs more than making parts strong enough to do the job out of cast iron. Does it not strike anyone as odd that you would want to pay more to get a light tractor made with aluminum assemblies and then pay yet more to add weights to get it heavy enough to do the work? Why?
Maybe for lawnmower duty aluminum would be good as less weight would not rut so easily and not tear up the turf so much when mowing the lawn. So there is likely a niche or two for lightweight tractors.
I am open minded though and would be pleased to hear anyone's argument as to why lighter is better in the vast majority of cases where tractors are employed to do more than mow the lawn. Please carefully explain the part about the advantages of paying extra to get a light weight tractor and then paying extra again to weight it so it can do real work. It can't be that someone just woke up and realized that heat transfer was a big tractor problem that had gone unrecognized for decades or that tractors are too heavy for most tasks.
Pat