I know the official reason is that it is hard to make, but chances are, even unprocessed aluminium would be much harder to locate with steel or iron burning, which I assume is the most common technique for prospecting.
Steel or Iron burning prospectors would easily find veins of other metals, making mining significantly easier, but this would not work for aluminium ores.
Unless the unprocessed forms can be traced easily, in which case I am completely wrong.