The best explanation I've heard is that it comes from the timeline of when different metals were discovered [2], back in the 1700s. Magnesium was discovered, and was named after Magnesia, a region in Greece. Basically, "magnesi-" + "-um". Then Barium was discovered, and was named from the Greek "baryta", meaning "heavy". Molybdenum followed the "-um" trend, and "tungsten"/"wolfram" ignored the discussion altogether.
And now we get to the first mistake. Tellurium was named from the Greek "tellus", and should have been called "tellurum". Instead, by analogy from magnesium and barium, the suffix "-ium" was added instead. Strontium came next, after the town of Strontian. This reinforced the trend of the "-ium" suffix, even though it came from "stronti-" + "-um". By the time Zirconium was discovered from the mineral "zircon", the association was cemented. So many metals had ended with "-ium" by sheer coincidence, that people assumed that that was the correct Latin suffix.
Bringing it back to the "aluminum" vs "aluminium", this means that there's neither authorial intent nor historical consistency in the use of "aluminium". I'm fine if people think it sounds better, and argue from that, but for the love of all that is holy, people need to quit pretended that it has any more validity or correctness to it.
[0] https://en.wikipedia.org/wiki/Aluminium#Coinage [1] https://en.wikipedia.org/wiki/Metals_of_antiquity [2] https://en.wikipedia.org/wiki/Timeline_of_chemical_element_d...
Why I am grouping "gif", "png", and "alumin[i]um" together here is that people consider some nebulous authority to be the prescriber of all things pronunciation. I personally think that pronunciation is derived from how it is commonly spoken, and not the other way around. Hence its inclusion in my playful ribbing. If I appeal to the correct authority, can I make them change their pronunciation? Maybe it wasn't very funny.