Mr.Webster of dictionary fame decided to only use Aluminum in his publication so that took over.

The bigger player here is probably Charles Martin Hall, who invented* a cheap method of refining it. Turns out yeah, if it's marketed and sold as aluminum in the US that's what people will know it as.

Although I guess it is possible Webster's dictionary influenced Hall's naming choice.

I wouldn't mind if we went back to calling it alumium, though.

* as a sidenote, also invented in France in the same year (coincidence) by Paul Héroult, thus called the Hall-Héroult process