Or as Americans like to call it, Titanum.
In other words, the British spelling was just a matter of British cultural hegemony at the time, just as American spellings are matters of American cultural hegemony today.
http://www.worldwidewords.org/articles/aluminium.htm
I didn't know any of this until just now.
"Sir Humphry made a bit of a mess of naming this new element, at first spelling it alumium (this was in 1807) then changing it to aluminum, and finally settling on aluminium in 1812."
To me it reads that British academics (Sir Humphry's peers) prefered the -ium spelling. It doesn't say they used political pressure to force him to change it.
While the use of both spellings were in almost equal use in the general British and US populations for a long time, the article hypothesises that the -um spelling became dominant in the US because of Webster's -um entry.