Why aren't aluminum electrodes allowed for use in grounding?

Al is a corossive metal...so cannot be used

Aluminum electrode conductors allowed cannot be in contact with earth, masonry, or subjected to corrosive conditions. When used outdoors, the termination to the electrode must not be within 18 in. of earth.

for good earthing the resistance should be as low as possible and material should also be corrosion free. That's why we prefer copper due to its low resistance,.