Aluminium flux-core wire has been tried in the past. First place I did an internship used to build the nose on Bombardier's trains, and the frame supporting the FG was made of welded aluminum. I know they tried some AL flux-core (from ESAB, I think but don't quote me on that). It worked with a spool gun, but they were having problems with weld quality (don't remember what it was).
What really killed it was the monetary aspect. The time saved by using flux-core was lessened by the more frequent spool changes, and that wire was expensive.
But with the developments in flux-core tech, I wouldn't be surprised to see someone try an aluminum flux-core wire again. They are starting to get good results with SS, and I personally think that flux-core should be the method of choice for roll-cage building nowadays. Mini rant over.