One reason that alum oxidizes quickly in electrical applications is being in proximity to other metals. They pass electrons to each other in a form of electrolisis, that accelerates oxidation. If it was all alum, it's not a problem, but since there is always some other steel around, you will get oxidation.
Bury a piece of alum wire and it will stay shinny for a long time. Bury it with some carbon steel nails and it will oxidize very quickly.
This is why boat builders try to make sure all the hardware/rigging on a boat is all the same type of metal. Either alum, zinc, steel, etc. You don't want to mix metals in the presence of sea water or electrolisis oxidation will occur.