I was just reading about Devils Tower for the o-wyoming list--apparently it's a mapmaking convention to ignore apostrophes (and other punctuation) in names (hence "Devils Tower" instead of "Devil's Tower"). Could that have something to do with it?
The apostrophe, which represents a glottal stop, is often omitted. I've always figured that that's because English speakers don't really have a concept of a glottal stop, so they overlook it and leave it out.
But maybe that's an insufficient explanation. Sure, English doesn't have glottal stops, but English also doesn't have the vowel combination /aɪiː/ ("ai-ee") and yet most Americans don't have any trouble pronouncing /həˈwaɪiː/ ("huh-wai-ee").
If both of these sounds, the glottal stop and the vowel combination, are absent in English, why do English speakers spurn the one and embrace the other?