[Swordmaker]

That's because diacritical marks have fallen out of use in English, so we don't have a means to indicate the accent on the back half of the diaeresis. Preëminent was the preferred spelling in the 19th and early 20th century, and the accent was also used in words like naïve and coöperation.

Exactly... When printers started simplifying their fonts (trays of type, not type faces) the diacritical marks were the first thing that were let go, it was a sorting nightmare and too easy to make a typesetting error... the hyphen in double letters with prefixes was frequently substituted and acceptable.

Frankly, the dropping of the hyphen is causing more confusion, in my opinion. I've seen people who read aloud stumble over the double vowels... saying pree minent... and coo peration... only to have to stop, and correct themselves. I'd prefer to add the hyphen to preserve the prefix separation.

Don't try to tell Flint that. I've been told not to address him anymore. He thinks he's above it...

[antiRepublicrat]

There’s also a general pattern in English where new formulations of words start with hyphens or other marks, and then the marks eventually disappear after the new word becomes common. It can happen rather quickly. The AP style guide says to insert a hyphen for “e” standing for “electronic” something in a word. But the exception is email. The word is so common in the culture now that the hyphen has disappeared after only about 20 years of common use.